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>

vitest.config.ts

@@ -0,0 +1,17 @@
+import { resolve } from 'path'
+import { defineConfig } from 'vitest/config'
+
+// Stand-alone vitest config. The unit tests only exercise the pure arg-building
+// logic in src/main/buildArgs.ts, which imports types/consts from @shared — so we
+// just need that one alias mirrored from electron.vite.config.ts.
+export default defineConfig({
+  resolve: {
+    alias: {
+      '@shared': resolve('src/shared')
+    }
+  },
+  test: {
+    include: ['test/**/*.test.ts'],
+    environment: 'node'
+  }
+})