Add Void Linux packaging; fix production builds embedding the frontend

Voiced now ships as a native Void Linux package:
- packaging/void/template: xbps-src template
- packaging/void/build-xbps.sh: builds and indexes a local .xbps repository
- packaging/void/voiced.desktop: application entry

Also fixes a real bug: the `custom-protocol` feature was never enabled, so
releases ran in development mode and tried to reach a dev server on
localhost:1420 instead of serving the embedded frontend. It is now declared
in Cargo.toml and passed explicitly by every build script.

The Void package output moved out of Vite's `dist/` (which is wiped on each
build) to `build/void/`.

Co-authored-by: CommandCodeBot <noreply@commandcode.ai>

.gitignore

@@ -4,6 +4,7 @@ node_modules/
 # Build output
 dist/
 dist-ssr/
+build/
 
 # Rust / Tauri
 src-tauri/target/