refactor: restructure to standard Go project layout (cmd/ + internal/)

- Move entry point to cmd/streamdeck-lets-go/
- Split package main into internal packages:
  - internal/deck: hardware control (Deck, events, brightness)
  - internal/render: key rendering, icons, fonts, PageManager
  - internal/web: HTTP API + embedded SPA
  - internal/daemon: event loop, autoswitch, screensaver, actions
- Add ConfigDir() to internal/config
- Export cross-package API (Deck, PageManager, WebServer, etc.)
- Move dist/arch/ → packaging/ with updated PKGBUILD/.SRCINFO
- Add Makefile (build, test, vet, fmt, install)
- Update README with new build commands and project structure
- Delete unused media.go stub

packaging/streamdeck-lets-go.service

@@ -0,0 +1,14 @@
+[Unit]
+Description=Stream Deck Lets Go daemon
+Documentation=https://github.com/Wakatron/streamdeck-lets-go
+After=graphical-session.target
+Wants=graphical-session.target
+
+[Service]
+Type=simple
+ExecStart=/usr/bin/streamdeck-lets-go daemon
+Restart=on-failure
+RestartSec=3
+
+[Install]
+WantedBy=default.target