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/VirtualSegmentList.svelte

@@ -1,4 +1,5 @@
-<script>
+<script lang="ts">
+  import type { ViewerSegment } from "$lib/types/app";
   import { measureTextHeight } from "$lib/utils/textMeasure.js";
   import { buildCumulativeOffsets, findVisibleRange } from "$lib/utils/virtualList.js";
   import { getPreferences } from "$lib/stores/preferences.svelte.js";
@@ -8,23 +9,39 @@
   let {
     segments = [],
     activeSegmentIdx = -1,
-    matchingSegments = new Set(),
+    matchingSegments = new Set<number>(),
     showTimestamps = true,
     editingIdx = -1,
     editingText = $bindable(""),
     searchQuery = "",
-    formatTime = (t) => String(t),
-    onSeekToTime = () => {},
-    onStartEditing = () => {},
+    formatTime = (t: number) => String(t),
+    onSeekToTime = (_time: number) => {},
+    onStartEditing = (_idx: number) => {},
     onFinishEditing = () => {},
-    onToggleStar = () => {},
-    onCopySegment = () => {},
-    onDeleteSegment = () => {},
-    highlightText = (t) => t,
-  } = $props();
+    onToggleStar = (_idx: number) => {},
+    onCopySegment = (_idx: number) => {},
+    onDeleteSegment = (_idx: number) => {},
+    highlightText = (t: string) => t,
+  } = $props<{
+    segments?: ViewerSegment[];
+    activeSegmentIdx?: number;
+    matchingSegments?: Set<number>;
+    showTimestamps?: boolean;
+    editingIdx?: number;
+    editingText?: string;
+    searchQuery?: string;
+    formatTime?: (time: number) => string;
+    onSeekToTime?: (time: number) => void;
+    onStartEditing?: (idx: number) => void;
+    onFinishEditing?: () => void;
+    onToggleStar?: (idx: number) => void;
+    onCopySegment?: (idx: number) => void;
+    onDeleteSegment?: (idx: number) => void;
+    highlightText?: (text: string) => string;
+  }>();
 
   // Internal state
-  let containerEl = $state(null);
+  let containerEl = $state<HTMLDivElement | null>(null);
   let containerHeight = $state(0);
   let containerWidth = $state(0);
   let scrollTop = $state(0);
@@ -46,7 +63,7 @@
     const textWidth = containerWidth - 26 - timestampWidth - 24;
     if (textWidth <= 0) return segments.map(() => 40);
 
-    return segments.map(seg => {
+    return segments.map((seg: ViewerSegment) => {
       const text = seg.text?.trim() || "";
       if (!text) return SEGMENT_PADDING + textLineHeight;
       const { height } = measureTextHeight(text, textFont, textWidth, textLineHeight);
@@ -75,7 +92,7 @@
 
   // Visible segment slice with positioning
   let visibleItems = $derived.by(() => {
-    const items = [];
+    const items: Array<{ seg: ViewerSegment; idx: number; top: number; height: number }> = [];
     for (let i = visibleRange.start; i < visibleRange.end; i++) {
       items.push({
         seg: segments[i],
@@ -88,7 +105,7 @@
   });
 
   // Scroll to a specific segment index (used for active segment tracking + search)
-  export function scrollToSegment(idx) {
+  export function scrollToSegment(idx: number) {
     if (!containerEl || idx < 0 || idx >= segments.length) return;
     const segTop = cumulativeOffsets[idx];
     const segHeight = segmentHeights[idx];
@@ -96,8 +113,8 @@
     containerEl.scrollTop = segTop - viewCenter + segHeight / 2;
   }
 
-  function onScroll(e) {
-    scrollTop = e.target.scrollTop;
+  function onScroll(e: Event) {
+    scrollTop = (e.currentTarget as HTMLDivElement).scrollTop;
     // Track selection state for expanded buffer
     hasSelection = !document.getSelection()?.isCollapsed;
   }
@@ -105,7 +122,7 @@
   // ResizeObserver for container dimensions
   $effect(() => {
     if (!containerEl) return;
-    const ro = new ResizeObserver(entries => {
+    const ro = new ResizeObserver((entries) => {
       for (const entry of entries) {
         containerHeight = entry.contentRect.height;
         containerWidth = entry.contentRect.width;
@@ -119,7 +136,7 @@
   $effect(() => {
     if (activeSegmentIdx >= 0) {
       // Find the index in our segments array that matches
-      const arrayIdx = segments.findIndex(s => s._idx === activeSegmentIdx);
+      const arrayIdx = segments.findIndex((s: ViewerSegment) => s._idx === activeSegmentIdx);
       if (arrayIdx >= 0) {
         const segTop = cumulativeOffsets[arrayIdx];
         const segBottom = segTop + segmentHeights[arrayIdx];