refactor(app): migrate main + preload to TypeScript with typed seams (Phase 5d-3c-ii)

Convert the app's main and preload source from .js to .ts (git mv preserves
history), adding types where they carry value: the core-consumption seam
(BuildIndexOptions/BuildIndexResult, FileInfo), the indexer service/worker
factories, and the preload IPC bridge. Module-to-module specifiers use the
real .ts extension (mirroring scripts/, since Node type-stripping does not
rewrite .js->.ts); the worker's runtime path stays indexer-worker.js because
that is the built output.

Toolchain:
- Add app/tsconfig.json: strict but noImplicitAny:false (the app orchestrates
  the already-strict core; annotating every SQLite-handle helper is low-value
  churn) + allowImportingTsExtensions (safe under noEmit).
- Add @types/better-sqlite3 for the injected binding.
- electron.vite.config.ts inputs -> .ts; refresh the stale CommonJS comment.
- typecheck script runs root + app projects. Root tsconfig excludes the
  app-importing tests (app-*.test.mjs, recap-capture-query.test.mjs) so the
  lenient app files are not dragged into the strict root program; the app
  source is covered by app/tsconfig.json instead. See docs/adr/0005.

Verified: npm run typecheck (root + app) clean; suite 121/121; electron-vite
build emits all 6 main entries + preload with no .ts/node:sqlite residue in
the bundles.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/tsconfig.json

@@ -0,0 +1,22 @@
+{
+  "compilerOptions": {
+    "target": "ES2022",
+    "module": "NodeNext",
+    "moduleResolution": "NodeNext",
+    "lib": ["ES2023"],
+    "types": ["node"],
+    "strict": true,
+    "noImplicitAny": false,
+    "noEmit": true,
+    "allowImportingTsExtensions": true,
+    "allowJs": true,
+    "checkJs": false,
+    "erasableSyntaxOnly": true,
+    "skipLibCheck": true,
+    "esModuleInterop": true,
+    "resolveJsonModule": true,
+    "forceConsistentCasingInFileNames": true
+  },
+  "include": ["src/main/**/*", "src/preload/**/*"],
+  "exclude": ["node_modules", "out", "dist", "release", "src/renderer"]
+}