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/stores/toasts.svelte.ts

@@ -0,0 +1,90 @@
+import type { ToastItem, ToastSeverity } from "$lib/types/app";
+import { errorMessage } from "$lib/utils/errors.js";
+
+// 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: ToastSeverity): number {
+  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 = $state<ToastItem[]>([]);
+
+  function show(severity: ToastSeverity, title: string, body?: string) {
+    const id = nextId++;
+    const duration = defaultDuration(severity);
+    const toast = { id, severity, title, body: body ?? '', duration };
+    items.push(toast);
+    if (duration > 0) {
+      setTimeout(() => dismiss(id), duration);
+    }
+    return id;
+  }
+
+  function dismiss(id: number) {
+    const ix = items.findIndex(t => t.id === id);
+    if (ix >= 0) items.splice(ix, 1);
+  }
+
+  function dismissAll() {
+    items.length = 0;
+  }
+
+  return {
+    get items() { return items; },
+    info: (title: string, body?: string) => show("info", title, body),
+    success: (title: string, body?: string) => show("success", title, body),
+    warn: (title: string, body?: string) => show("warn", title, body),
+    error: (title: string, body?: string) => show("error", title, body),
+    dismiss,
+    dismissAll,
+  };
+}
+
+export const toasts = createToastsStore();
+
+// Helper: wrap a Tauri invoke and toast on failure. Returns the result on
+// success, throws on failure (so callers that need to handle the error
+// can still try/catch).
+//
+//   import { invoke } from '@tauri-apps/api/core';
+//   import { invokeWithToast } from '$lib/stores/toasts.svelte.js';
+//   const result = await invokeWithToast('start_native_capture', { deviceName });
+//
+// Optional `errorTitle` overrides the default ("Action failed").
+export async function invokeWithToast<TArgs, TResult>(
+  invokeFn: (command: string, args?: TArgs) => Promise<TResult>,
+  command: string,
+  args?: TArgs,
+  errorTitle?: string,
+): Promise<TResult> {
+  try {
+    return await invokeFn(command, args);
+  } catch (err) {
+    toasts.error(errorTitle ?? "Action failed", errorMessage(err));
+    throw err;
+  }
+}