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>

src-tauri/Cargo.toml

@@ -26,6 +26,12 @@ uuid = { version = "1", features = ["v4"] }
 thiserror = "2"
 base64 = "0.22"
 
+[features]
+# Production builds embed the frontend and serve it over the custom asset
+# protocol. Without this, the app expects a dev server on localhost:1420.
+# DO NOT REMOVE.
+custom-protocol = ["tauri/custom-protocol"]
+
 [profile.release]
 codegen-units = 1
 lto = true