ui: Day 3 — global toast system + first error-toast wiring on DictationPage

THE PROBLEM (Codex review + architecture-review.md §1, §14):
Multiple critical paths discard errors silently — `let _ = insert_transcript(...)`,
`.catch(() => {})`, inline `error = ...` state that not every page surfaces. ND
users in particular need explicit feedback when something fails; silent
failure is worse than no feature.

SHIPPED:

src/lib/stores/toasts.svelte.js (new):
- Minimal in-house toast store (~80 lines, no svelte-french-toast dep)
- Severity → palette: info/success → moss, warn → signal, error → ember
- Auto-dismiss durations per severity (success 3s, info 4s, warn 6s,
  error sticky)
- `invokeWithToast(invoke, command, args, errorTitle?)` helper wraps a
  Tauri invoke and toasts on failure while still throwing for callers
  that need to handle the error themselves

src/lib/components/ToastViewport.svelte (new):
- Reads from the toasts store
- Bottom-right stack, max-width clamp on small screens
- aria-live=polite + role=alert on error toasts (screen-reader friendly)
- Slide-in animation honours `html.reduce-motion` for the existing
  preferences toggle
- Severity left-border in brand colours via existing CSS variables

src/routes/+layout.svelte:
- Mounts <ToastViewport /> once at the app root so toasts work from any
  page or window

src/lib/pages/DictationPage.svelte:
- First error-toast wiring: failures starting recording now show a
  sticky toast in addition to the inline `error` panel
- Replaces a previously easy-to-miss failure-mode

NEXT (Day 4 batch):
- Wire toasts into FilesPage, HistoryPage, SettingsPage, ModelDownloader
  (all currently swallow errors)
- Add `add_transcript`, `list_transcripts`, `update_transcript` (closes the
  long-standing TODO Codex flagged), `delete_transcript` Tauri commands
  wrapping the existing storage layer
- Switch addToHistory to dual-write (localStorage + SQLite) so the
  canonical store catches up with the actual transcript flow
- FTS5 transcripts_fts virtual table + search command
- Wrap multi-row writes (decompose_and_store) in DB transactions

src/lib/stores/toasts.svelte.js

@@ -0,0 +1,83 @@
+// Minimal toast store. Roll-our-own (no svelte-french-toast etc) so Kon
+// stays lean. Toasts auto-dismiss after `duration` ms unless duration is 0
+// (sticky) or unless the user clicks the close button.
+//
+// Severity → colour:
+//   info    → moss      (informational, default 4s)
+//   success → moss      (operation succeeded, default 3s)
+//   warn    → signal    (degraded but not failed, default 6s)
+//   error   → ember     (operation failed, sticky until dismissed)
+//
+// Usage from a component:
+//   import { toasts } from '$lib/stores/toasts.svelte.js';
+//   toasts.error('Could not start recording', 'Selected microphone disappeared.');
+//   toasts.success('Saved');
+
+let nextId = 1;
+
+function defaultDuration(severity) {
+  switch (severity) {
+    case 'success': return 3000;
+    case 'info':    return 4000;
+    case 'warn':    return 6000;
+    case 'error':   return 0; // sticky
+    default:        return 4000;
+  }
+}
+
+function createToastsStore() {
+  // $state requires a class field or top-level let in Svelte 5; we expose
+  // `items` via a getter on the singleton.
+  let items = $state([]);
+
+  function show(severity, title, body) {
+    const id = nextId++;
+    const duration = defaultDuration(severity);
+    const toast = { id, severity, title, body: body ?? '', duration };
+    items.push(toast);
+    if (duration > 0) {
+      setTimeout(() => dismiss(id), duration);
+    }
+    return id;
+  }
+
+  function dismiss(id) {
+    const ix = items.findIndex(t => t.id === id);
+    if (ix >= 0) items.splice(ix, 1);
+  }
+
+  function dismissAll() {
+    items.length = 0;
+  }
+
+  return {
+    get items() { return items; },
+    info:    (title, body) => show('info',    title, body),
+    success: (title, body) => show('success', title, body),
+    warn:    (title, body) => show('warn',    title, body),
+    error:   (title, body) => show('error',   title, body),
+    dismiss,
+    dismissAll,
+  };
+}
+
+export const toasts = createToastsStore();
+
+// Helper: wrap a Tauri invoke and toast on failure. Returns the result on
+// success, throws on failure (so callers that need to handle the error
+// can still try/catch).
+//
+//   import { invoke } from '@tauri-apps/api/core';
+//   import { invokeWithToast } from '$lib/stores/toasts.svelte.js';
+//   const result = await invokeWithToast('start_native_capture', { deviceName });
+//
+// Optional `errorTitle` overrides the default ("Action failed").
+export async function invokeWithToast(invokeFn, command, args, errorTitle) {
+  try {
+    return await invokeFn(command, args);
+  } catch (err) {
+    const msg = typeof err === 'string' ? err : (err?.message ?? String(err));
+    toasts.error(errorTitle ?? 'Action failed', msg);
+    throw err;
+  }
+}