refactor: migrate from Codeberg to Gitea and rename rhemawave to radiantwave

- Update all Codeberg URLs to gitea.radiantwavetech.com/RadiantWaveTech/
- Rename flake inputs: rhemawave → radiantwave, rhemawave-dev → radiantwave-dev
- Rename NixOS configs: rhemawavekiosk* → radiantwavekiosk*
- Update systemd services, data paths, and binary references
- Rename plymouth theme files
- Update CI workflow, documentation, and build scripts

build-rpi.sh

@@ -1,12 +1,12 @@
 #!/usr/bin/env bash
 set -euo pipefail
 
-CONFIG="rhemawavekiosk-rpi"
+CONFIG="radiantwavekiosk-rpi"
 VARIANT=""
 
 for arg in "$@"; do
     case "$arg" in
-        --dev) CONFIG="rhemawavekiosk-rpi-dev"; VARIANT="-dev" ;;
+        --dev) CONFIG="radiantwavekiosk-rpi-dev"; VARIANT="-dev" ;;
         *) echo "Unknown arg: $arg"; exit 1 ;;
     esac
 done