Add a pure, unit-tested buildArgs() module

Extracts yt-dlp argv construction out of download.ts into its own
electron-free module (src/main/buildArgs.ts) so it can be exercised by
vitest without spinning up Electron, plus a real-download integration
test (skipped by default; opt in with AEROFETCH_REAL_DOWNLOAD=1) that
spawns the bundled yt-dlp.exe/ffmpeg.exe against its argv. download.ts
isn't wired up to it yet — that lands with the next commit.

This was done in an earlier local session but never actually committed,
even though download.ts has imported from it since the Phase A commit
(bb5dd6c) — the build had been relying on an uncommitted local file.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

package.json

@@ -18,7 +18,8 @@
     "start": "electron-vite preview",
     "typecheck:node": "tsc --noEmit -p tsconfig.node.json --composite false",
     "typecheck:web": "tsc --noEmit -p tsconfig.web.json --composite false",
-    "typecheck": "npm run typecheck:node && npm run typecheck:web"
+    "typecheck": "npm run typecheck:node && npm run typecheck:web",
+    "test": "vitest run"
   },
   "dependencies": {
     "@electron-toolkit/utils": "^4.0.0",
@@ -39,6 +40,7 @@
     "electron-builder": "^26.15.3",
     "electron-vite": "^5.0.0",
     "typescript": "^6.0.3",
-    "vite": "^7.3.5"
+    "vite": "^7.3.5",
+    "vitest": "^4.1.9"
   }
 }