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/virtualList.ts

@@ -0,0 +1,49 @@
+export function buildCumulativeOffsets(heights: number[]): number[] {
+  const offsets = new Array(heights.length + 1);
+  offsets[0] = 0;
+  for (let i = 0; i < heights.length; i++) {
+    offsets[i + 1] = offsets[i] + heights[i];
+  }
+  return offsets;
+}
+
+export function findVisibleRange(
+  cumulativeOffsets: number[],
+  itemCount: number,
+  scrollTop: number,
+  viewportHeight: number,
+  buffer = 4,
+) {
+  if (!cumulativeOffsets.length || itemCount === 0 || viewportHeight <= 0) {
+    return { start: 0, end: 0 };
+  }
+
+  let lo = 0;
+  let hi = itemCount - 1;
+  while (lo < hi) {
+    const mid = (lo + hi) >>> 1;
+    if (cumulativeOffsets[mid + 1] < scrollTop) {
+      lo = mid + 1;
+    } else {
+      hi = mid;
+    }
+  }
+  const start = Math.max(0, lo - buffer);
+
+  const endScroll = scrollTop + viewportHeight;
+  lo = start;
+  hi = itemCount - 1;
+  while (lo < hi) {
+    const mid = (lo + hi) >>> 1;
+    if (cumulativeOffsets[mid] < endScroll) {
+      lo = mid + 1;
+    } else {
+      hi = mid;
+    }
+  }
+
+  return {
+    start,
+    end: Math.min(itemCount, lo + buffer),
+  };
+}