refactor(frontend): migrate JS modules to TypeScript

Wholesale JS -> TS migration of the frontend — stores, utils, actions,
and all Svelte component scripts adopt type annotations. Compile-time
surfaces (app.d.ts, lib/types/) added for shared DTO types.

Build plumbing:
  - package.json: dev:frontend script that runs svelte-kit sync first
  - tauri.conf.json: beforeDevCommand points at dev:frontend
  - run.sh: dropped the sed-hack that temporarily blanked beforeDevCommand;
    now relies on npm run dev:frontend to avoid double-Vite
  - jsconfig.json: allowImportingTsExtensions

Preserves all Group 1 behaviour:
  - page.svelte.ts keeps loadHistory / loadTasks Tauri-first, no
    localStorage; saveTranscriptMeta + mapTranscriptRow + mapTaskRow
    intact; update_task_cmd and update_transcript_meta_cmd invocations
    carry the correct payload shape.
  - Toasts, preferences stores typed without behaviour change.
  - Viewer still routes segment edits through saveTranscriptMeta; the
    Task 1.5 TODO markers are gone.

taskExtractor.ts is functionally improved during the migration:
  - multi-task matches in the same sentence
  - list-style shopping-verb expansion (get bread, milk, and cheese)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/lib/utils/osInfo.ts

@@ -0,0 +1,110 @@
+// Cross-platform OS info for the Svelte frontend. Single async fetch,
+// cached in memory, exposes synchronous helpers after first await.
+//
+// Backed by the `get_os_info` Tauri command (commands/diagnostics.rs).
+// In browser-preview mode (no Tauri runtime), falls back to a sensible
+// default based on navigator.platform.
+//
+// Usage:
+//   import { loadOsInfo, osInfo, modKeyLabel, isMac } from '$lib/utils/osInfo.js';
+//   await loadOsInfo();          // call once at app startup
+//   modKeyLabel();               // "Cmd" on macOS, "Ctrl" elsewhere
+//   if (isMac()) { ... }
+//
+// The store version (osInfo) is reactive — Svelte components can read it
+// directly with `$osInfo` once Svelte 5 runes are everywhere, or via
+// destructuring on a non-reactive read.
+
+import { invoke } from '@tauri-apps/api/core';
+import { hasTauriRuntime } from './runtime.js';
+
+interface OsInfo {
+  os: string;
+  arch: string;
+  family: string;
+  usesCmd: boolean;
+  isWayland: boolean;
+  customHotkeyBackend: boolean;
+  primaryModifierLabel: string;
+}
+
+let cached: OsInfo | null = null;
+
+const FALLBACK_BROWSER_INFO = {
+  os: detectBrowserOs(),
+  arch: 'unknown',
+  family: detectBrowserFamily(),
+  usesCmd: detectBrowserOs() === 'macos',
+  isWayland: false,
+  customHotkeyBackend: false,
+  primaryModifierLabel: detectBrowserOs() === 'macos' ? 'Cmd' : 'Ctrl',
+};
+
+function detectBrowserOs() {
+  if (typeof navigator === 'undefined') return 'unknown';
+  const ua = (navigator.userAgent || '').toLowerCase();
+  const platform = (navigator.platform || '').toLowerCase();
+  if (platform.includes('mac') || ua.includes('mac os')) return 'macos';
+  if (platform.includes('win') || ua.includes('windows')) return 'windows';
+  if (platform.includes('linux') || ua.includes('linux')) return 'linux';
+  return 'unknown';
+}
+
+function detectBrowserFamily() {
+  switch (detectBrowserOs()) {
+    case 'macos': return 'macOS';
+    case 'windows': return 'Windows';
+    case 'linux': return 'Linux';
+    default: return 'Unknown';
+  }
+}
+
+/** Fetch OS info from the Tauri backend. Idempotent — call multiple times,
+ *  only the first does the round-trip. Browser-preview mode returns the
+ *  navigator-detected fallback. */
+export async function loadOsInfo(): Promise<OsInfo> {
+  if (cached) return cached;
+  if (!hasTauriRuntime()) {
+    cached = FALLBACK_BROWSER_INFO;
+    return cached;
+  }
+  try {
+    cached = await invoke<OsInfo>("get_os_info");
+  } catch (err) {
+    console.warn('loadOsInfo: get_os_info failed, using browser fallback', err);
+    cached = FALLBACK_BROWSER_INFO;
+  }
+  return cached;
+}
+
+/** Synchronous accessor. Returns null if loadOsInfo has not been awaited
+ *  yet. Components that need the value at first render should await
+ *  loadOsInfo() in onMount before reading. */
+export function osInfo(): OsInfo | null {
+  return cached;
+}
+
+export function isMac(): boolean {
+  return cached?.os === 'macos';
+}
+
+export function isWindows(): boolean {
+  return cached?.os === 'windows';
+}
+
+export function isLinux(): boolean {
+  return cached?.os === 'linux';
+}
+
+/** Localised label for the primary keyboard modifier. "Cmd" on macOS,
+ *  "Ctrl" everywhere else. Use in hotkey display strings:
+ *      `${modKeyLabel()}+Shift+R` */
+export function modKeyLabel(): string {
+  return cached?.primaryModifierLabel ?? 'Ctrl';
+}
+
+/** True if the current Linux session is Wayland. False on Windows/macOS.
+ *  Useful for Settings → Audio "PipeWire detected" status display. */
+export function isWayland(): boolean {
+  return !!cached?.isWayland;
+}