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/components/WipTaskList.svelte

@@ -1,7 +1,13 @@
 <script lang="ts">
   import { tasks, addTask, completeTask, uncompleteTask, deleteTask } from '$lib/stores/page.svelte.js';
   import MicroSteps from '$lib/components/MicroSteps.svelte';
-  import { ChevronDown, ChevronRight } from 'lucide-svelte';
+  import { ChevronDown, ChevronRight, Timer } from 'lucide-svelte';
+
+  function startFocusTimer(task: { id: string; text: string }) {
+    window.dispatchEvent(new CustomEvent('kon:start-timer', {
+      detail: { taskId: task.id, seconds: 300, label: task.text }
+    }));
+  }
 
   let { wipLimit = 3 } = $props();
 
@@ -68,6 +74,16 @@
               aria-label="Complete task"
             ></button>
             <span class="text-[13px] text-text flex-1 min-w-0 truncate">{task.text}</span>
+            <!-- 5-min focus timer — the "just-start" button from the brief -->
+            <button
+              class="opacity-0 group-hover:opacity-100 text-text-tertiary hover:text-accent"
+              onclick={() => startFocusTimer(task)}
+              aria-label="Start 5-minute focus timer for this task"
+              title="Start 5-minute focus timer"
+              style="transition: opacity var(--duration-ui)"
+            >
+              <Timer size={12} aria-hidden="true" />
+            </button>
             <!-- Expand/collapse micro-steps toggle -->
             <button
               class="opacity-0 group-hover:opacity-100 text-text-tertiary hover:text-accent"