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>

packaging/void/voiced.desktop

@@ -0,0 +1,13 @@
+[Desktop Entry]
+Type=Application
+Version=1.0
+Name=Voiced
+GenericName=Invoice Generator
+Comment=Offline invoicing by Bongbetic
+Exec=voiced
+Icon=voiced
+Terminal=false
+Categories=Office;Finance;
+Keywords=invoice;billing;gst;pdf;bongbetic;
+StartupNotify=true
+StartupWMClass=Voiced