feat(config+ux): batch #5 — source maps, integration script, copy, redownload options

Clean config/copy items (Phase 7):
- L170: build.sourcemap 'hidden' on main/preload/renderer — emit .map for offline
  crash symbolication (of the CC8 logger's raw stacks) without exposing them.
- L34: add `test:integration` npm script (AEROFETCH_REAL_DOWNLOAD=1) for the opt-in
  real-download suite; stays out of `npm test` (needs network + live yt-dlp).
- L87: HistoryEntry carries the original `options`; redownload replays them so a
  re-download reproduces the same post-processing instead of the current defaults.
- L96: folder "Browse" buttons -> "Browse…" (they open the OS picker).
- L97: PO-token placeholder "(none)" -> "Optional -- paste a token" (consistent).
- Ticked already-resolved: L99 (M28 aria-label), L113 (UX2 tooltip=aria-label),
  L95 (usage follows verb/modifier rule), L172 (skipLibCheck accepted).

typecheck + 268 tests + eslint + prettier green (touched files LF/clean).

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

package.json

@@ -29,7 +29,8 @@
     "lint:fix": "eslint . --fix",
     "format": "prettier --write \"src/**/*.{ts,tsx}\" \"test/**/*.ts\"",
     "format:check": "prettier --check \"src/**/*.{ts,tsx}\" \"test/**/*.ts\"",
-    "test": "vitest run"
+    "test": "vitest run",
+    "test:integration": "set AEROFETCH_REAL_DOWNLOAD=1&& vitest run test/real-download.integration.test.ts"
   },
   "dependencies": {
     "@electron-toolkit/utils": "^4.0.0",