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/components/ToastViewport.svelte

@@ -1,4 +1,5 @@
-<script>
+<script lang="ts">
+  import type { ToastSeverity } from "$lib/types/app";
   // Renders the toast stack in the bottom-right of the viewport. Mount once
   // at the app root (in +layout.svelte). Reads from the global toasts store.
   //
@@ -10,7 +11,7 @@
   import { toasts } from '$lib/stores/toasts.svelte.js';
   import { X } from 'lucide-svelte';
 
-  function severityClass(severity) {
+  function severityClass(severity: ToastSeverity) {
     switch (severity) {
       case 'success': return 'toast-success';
       case 'info':    return 'toast-info';