feat(focus-timer): Phase 1 — visual countdown ring + just-start timer

Closes the Core MVP gap in docs/brief/feature-set.md ("visual time
representation") and wires the dangling kon:start-timer emit that
MicroSteps.svelte has been firing into the void since the stub was
written. Implements phase 1 of the 2026-04-23 feature-complete
roadmap.

New:
- src/lib/stores/focusTimer.svelte.ts — singleton timer store with
  localStorage persistence so a timer started in Dictation survives
  page nav, window close, and reopen. Gentle WebAudio chime at
  completion (no bundled asset). 250 ms tick. Completion flash for
  3 s before auto-clear.
- src/lib/components/FocusTimer.svelte — floating top-right overlay
  with SVG progress ring (shrinking colour: accent -> warning in
  the final 15% -> success on completion). Cancel + "+1 min" on
  hover. Renders nothing when idle.

Wired in +layout.svelte next to ToastViewport.

Two triggers now in the app:
- MicroSteps row 2-min button (pre-existing emit, previously no
  listener)
- WipTaskList row 5-min button (new; the brief's "just-start"
  from the Now column)

Respects prefers-reduced-motion via the existing
[data-reduce-motion] attribute.

Out of scope, carried to later phases: rhythmic voice anchoring
(Phase 6), custom-duration picker, multi-timer UI, native OS
notification (deferred to Phase 6 with the full nudge pipeline).

src/lib/stores/focusTimer.svelte.ts

@@ -0,0 +1,230 @@
+// Focus timer store. Single active timer at a time — the "just-start"
+// 2/5/10/15-minute countdown paired with micro-steps. Exposes:
+//   - focusTimer.active: whether a timer is currently running
+//   - focusTimer.progress: 0..1 fraction of elapsed time
+//   - focusTimer.remainingMs: milliseconds until completion
+//   - focusTimer.label / focusTimer.taskId: what this timer is for
+//   - start(seconds, opts) / cancel() / extend(seconds)
+//
+// Survives window close + reopen via localStorage, because a timer
+// that loses its clock when the user alt-tabs is a timer that nobody
+// trusts. On rehydrate after expiry, fires completion then clears —
+// so closing the window mid-timer still gets you the "done" signal
+// on next launch.
+
+const STORAGE_KEY = "kon.focusTimer.v1";
+const TICK_INTERVAL_MS = 250;
+
+export type FocusTimerPersisted = {
+  startedAt: number;
+  durationMs: number;
+  taskId: string | null;
+  label: string | null;
+};
+
+function readPersisted(): FocusTimerPersisted | null {
+  if (typeof window === "undefined") return null;
+  try {
+    const raw = window.localStorage.getItem(STORAGE_KEY);
+    if (!raw) return null;
+    const parsed = JSON.parse(raw);
+    if (
+      typeof parsed?.startedAt !== "number" ||
+      typeof parsed?.durationMs !== "number"
+    ) return null;
+    return {
+      startedAt: parsed.startedAt,
+      durationMs: parsed.durationMs,
+      taskId: parsed.taskId ?? null,
+      label: parsed.label ?? null,
+    };
+  } catch {
+    return null;
+  }
+}
+
+function writePersisted(state: FocusTimerPersisted | null): void {
+  if (typeof window === "undefined") return;
+  try {
+    if (state === null) window.localStorage.removeItem(STORAGE_KEY);
+    else window.localStorage.setItem(STORAGE_KEY, JSON.stringify(state));
+  } catch { /* storage may be disabled; non-fatal */ }
+}
+
+function createFocusTimerStore() {
+  let startedAt = $state<number | null>(null);
+  let durationMs = $state<number>(0);
+  let taskId = $state<string | null>(null);
+  let label = $state<string | null>(null);
+  let now = $state<number>(Date.now());
+  let completionFlashUntil = $state<number>(0);
+
+  let interval: ReturnType<typeof setInterval> | null = null;
+
+  const active = $derived(startedAt !== null);
+  const elapsedMs = $derived(startedAt === null ? 0 : Math.max(0, now - startedAt));
+  const remainingMs = $derived(Math.max(0, durationMs - elapsedMs));
+  const progress = $derived(durationMs === 0 ? 0 : Math.min(1, elapsedMs / durationMs));
+  const finished = $derived(active && remainingMs === 0);
+  const showingCompletionFlash = $derived(now < completionFlashUntil);
+
+  // Internal completion + flash bookkeeping. We track whether we have
+  // already fired the completion chime for the current timer so a
+  // second tick does not re-fire it. Reset whenever a new timer starts.
+  let completionFired = false;
+
+  function tick() {
+    now = Date.now();
+    // Fire completion once, the first tick after we cross remaining=0.
+    if (startedAt !== null && !completionFired && now - startedAt >= durationMs) {
+      completionFired = true;
+      completionFlashUntil = now + 3000;
+      fireCompletion();
+    }
+    // After the 3 s flash window, clear everything and stop ticking.
+    if (completionFlashUntil > 0 && now >= completionFlashUntil) {
+      clear();
+    }
+  }
+
+  function startTick() {
+    if (interval !== null) return;
+    interval = setInterval(tick, TICK_INTERVAL_MS);
+  }
+
+  function stopTick() {
+    if (interval !== null) {
+      clearInterval(interval);
+      interval = null;
+    }
+  }
+
+  function clear() {
+    startedAt = null;
+    durationMs = 0;
+    taskId = null;
+    label = null;
+    completionFlashUntil = 0;
+    completionFired = false;
+    writePersisted(null);
+    stopTick();
+  }
+
+  function start(seconds: number, opts?: { taskId?: string | null; label?: string | null }): void {
+    if (!Number.isFinite(seconds) || seconds <= 0) return;
+    now = Date.now();
+    startedAt = now;
+    durationMs = Math.floor(seconds * 1000);
+    taskId = opts?.taskId ?? null;
+    label = opts?.label ?? null;
+    completionFlashUntil = 0;
+    completionFired = false;
+    writePersisted({ startedAt, durationMs, taskId, label });
+    startTick();
+  }
+
+  function cancel(): void {
+    clear();
+  }
+
+  function extend(seconds: number): void {
+    if (startedAt === null) return;
+    if (!Number.isFinite(seconds) || seconds <= 0) return;
+    durationMs += Math.floor(seconds * 1000);
+    writePersisted({ startedAt, durationMs, taskId, label });
+  }
+
+  function dismissCompletionFlash(): void {
+    completionFlashUntil = 0;
+    clear();
+  }
+
+  function fireCompletion(): void {
+    if (typeof window === "undefined") return;
+    // Gentle chime — WebAudio so we do not ship a bundled asset.
+    // A 440 Hz fall into 330 Hz over 220 ms, low volume, no sustain.
+    try {
+      type AudioCtx = typeof AudioContext;
+      const win = window as unknown as { AudioContext?: AudioCtx; webkitAudioContext?: AudioCtx };
+      const Ctx = win.AudioContext ?? win.webkitAudioContext;
+      if (!Ctx) return;
+      const ctx = new Ctx();
+      const osc = ctx.createOscillator();
+      const gain = ctx.createGain();
+      osc.type = "sine";
+      osc.frequency.setValueAtTime(440, ctx.currentTime);
+      osc.frequency.exponentialRampToValueAtTime(330, ctx.currentTime + 0.22);
+      gain.gain.setValueAtTime(0.0001, ctx.currentTime);
+      gain.gain.exponentialRampToValueAtTime(0.15, ctx.currentTime + 0.02);
+      gain.gain.exponentialRampToValueAtTime(0.0001, ctx.currentTime + 0.28);
+      osc.connect(gain);
+      gain.connect(ctx.destination);
+      osc.start();
+      osc.stop(ctx.currentTime + 0.3);
+      osc.onended = () => ctx.close().catch(() => {});
+    } catch { /* audio is a nicety; never fatal */ }
+
+    window.dispatchEvent(new CustomEvent("kon:focus-timer-complete", {
+      detail: { taskId, label },
+    }));
+  }
+
+  // Rehydrate on first touch. If the persisted timer has already
+  // expired, fire completion then clear so the user still gets the
+  // "done" signal they missed while the window was closed.
+  function rehydrate(): void {
+    const persisted = readPersisted();
+    if (!persisted) return;
+    const age = Date.now() - persisted.startedAt;
+    if (age >= persisted.durationMs) {
+      // Already expired while the window was closed. Fire a completion
+      // event so downstream listeners (nudges, UI flourishes) can react.
+      taskId = persisted.taskId;
+      label = persisted.label;
+      durationMs = persisted.durationMs;
+      startedAt = persisted.startedAt;
+      now = persisted.startedAt + persisted.durationMs;
+      // Flash briefly so the user knows it happened.
+      completionFlashUntil = Date.now() + 3000;
+      completionFired = true;
+      fireCompletion();
+      startTick();
+      return;
+    }
+    startedAt = persisted.startedAt;
+    durationMs = persisted.durationMs;
+    taskId = persisted.taskId;
+    label = persisted.label;
+    now = Date.now();
+    startTick();
+  }
+
+  // Exposed as frozen object. Getters so derivations stay reactive.
+  return {
+    get active() { return active; },
+    get progress() { return progress; },
+    get elapsedMs() { return elapsedMs; },
+    get remainingMs() { return remainingMs; },
+    get durationMs() { return durationMs; },
+    get taskId() { return taskId; },
+    get label() { return label; },
+    get showingCompletionFlash() { return showingCompletionFlash; },
+    start,
+    cancel,
+    extend,
+    rehydrate,
+    dismissCompletionFlash,
+  };
+}
+
+export const focusTimer = createFocusTimerStore();
+
+// Preset durations surfaced in the UI. 2 / 5 / 10 / 15 minutes match
+// the brief's guidance for the "just-start" timer and cover common
+// Pomodoro / shorter-focus preferences.
+export const FOCUS_TIMER_PRESETS_SECONDS: ReadonlyArray<{ label: string; seconds: number }> = [
+  { label: "2 min", seconds: 120 },
+  { label: "5 min", seconds: 300 },
+  { label: "10 min", seconds: 600 },
+  { label: "15 min", seconds: 900 },
+];