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/.SRCINFO

@@ -0,0 +1,23 @@
+pkgbase = streamdeck-lets-go-git
+	pkgdesc = Lightweight daemon for controlling Elgato Stream Deck devices with a built-in web UI
+	pkgver = r0.unknown
+	pkgrel = 1
+	url = https://github.com/totmin/streamdeck-lets-go
+	arch = x86_64
+	arch = aarch64
+	license = MIT
+	makedepends = go>=1.26
+	makedepends = git
+	depends = libusb
+	optdepends = librsvg: SVG icon rendering
+	optdepends = fontconfig: system font detection
+	provides = streamdeck-lets-go
+	conflicts = streamdeck-lets-go
+	source = streamdeck-lets-go-git::git+https://github.com/totmin/streamdeck-lets-go.git
+	source = packaging/streamdeck-lets-go.service
+	source = packaging/90-streamdeck.rules
+	b2sums = SKIP
+	b2sums = SKIP
+	b2sums = SKIP
+
+pkgname = streamdeck-lets-go-git