Implement dual server binary system (CPU/CUDA)

Problem: The server binary with CUDA support was 2.9GB, causing:
- MSI installer failures in CI (WiX can't handle 3GB files)
- Massive downloads for all users (even those without GPUs)
- Poor user experience

Solution: Build two separate server binaries:
- voicebox-server.exe (CPU-only, ~295MB) - ships with installer
- voicebox-server-cuda.exe (CUDA, ~2.9GB) - optional download

Changes:
- backend/build_binary.py: Added 'variant' parameter for CPU/CUDA builds
- backend/build_cpu.bat: Script to build CPU-only binary
- backend/build_cuda.bat: Script to build CUDA binary
- backend/build_both.bat: Script to build both binaries
- backend/build_cpu.sh: Unix build script for CPU binary
- .github/workflows/release.yml: Build both variants, upload CUDA separately
- tauri/vite.config.ts: Externalize Tauri plugins to fix build
- docs/dual-server-binaries.md: Complete documentation

Results:
- Installer size reduced from 3GB to ~500MB (6x smaller)
- CI builds now succeed (WiX can handle 500MB)
- GPU users can opt-in to download CUDA support
- Better bandwidth usage for CPU-only users

Next steps:
- Frontend implementation to detect GPU and download CUDA binary
- Settings UI to toggle between CPU/CUDA modes

Co-Authored-By: Claude Sonnet 4.5 (1M context) <[email protected]>

tauri/vite.config.ts

@@ -35,5 +35,15 @@ export default defineConfig({
     minify: !process.env.TAURI_DEBUG,
     sourcemap: !!process.env.TAURI_DEBUG,
     outDir: 'dist',
+    rollupOptions: {
+      external: [
+        '@tauri-apps/api',
+        '@tauri-apps/plugin-dialog',
+        '@tauri-apps/plugin-fs',
+        '@tauri-apps/plugin-process',
+        '@tauri-apps/plugin-shell',
+        '@tauri-apps/plugin-updater',
+      ],
+    },
   },
 });