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>

package.json

@@ -11,8 +11,8 @@
     "preview": "vite preview",
     "tauri": "tauri",
     "app:dev": "tauri dev",
-    "app:build": "tauri build",
-    "app:build:windows": "tauri build --runner cargo-xwin --target x86_64-pc-windows-msvc"
+    "app:build": "tauri build --features custom-protocol",
+    "app:build:windows": "tauri build --runner cargo-xwin --target x86_64-pc-windows-msvc --features custom-protocol"
   },
   "dependencies": {
     "@carbon/icons-react": "^11.89.0",