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/package-lock.json

@@ -12,6 +12,7 @@
         "chokidar": "^4.0.3"
       },
       "devDependencies": {
+        "@types/better-sqlite3": "^7.6.13",
         "@vitejs/plugin-vue": "^5.0.0",
         "electron": "^33.0.0",
         "electron-builder": "^25.0.0",
@@ -1848,6 +1849,16 @@
         "node": ">= 10"
       }
     },
+    "node_modules/@types/better-sqlite3": {
+      "version": "7.6.13",
+      "resolved": "https://registry.npmjs.org/@types/better-sqlite3/-/better-sqlite3-7.6.13.tgz",
+      "integrity": "sha512-NMv9ASNARoKksWtsq/SHakpYAYnhBrQgGD8zkLYk/jaK8jUGn08CfEdTRgYhMypUQAfzSP8W6gNLe0q19/t4VA==",
+      "dev": true,
+      "license": "MIT",
+      "dependencies": {
+        "@types/node": "*"
+      }
+    },
     "node_modules/@types/cacheable-request": {
       "version": "6.0.3",
       "resolved": "https://registry.npmjs.org/@types/cacheable-request/-/cacheable-request-6.0.3.tgz",