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/PKGBUILD

@@ -0,0 +1,48 @@
+# Maintainer: totmin <totmin@users.noreply.github.com>
+
+pkgname=streamdeck-lets-go-git
+_giturl=https://github.com/totmin/streamdeck-lets-go.git
+pkgver=0.1.0
+pkgrel=1
+pkgdesc="Lightweight daemon for controlling Elgato Stream Deck devices with a built-in web UI"
+arch=('x86_64' 'aarch64')
+url="https://github.com/totmin/streamdeck-lets-go"
+license=('MIT')
+depends=('libusb')
+optdepends=('librsvg: SVG icon rendering'
+            'fontconfig: system font detection')
+makedepends=('go>=1.26' 'git')
+provides=('streamdeck-lets-go')
+conflicts=('streamdeck-lets-go')
+
+source=("${pkgname}::git+${_giturl}"
+        "packaging/streamdeck-lets-go.service"
+        "packaging/90-streamdeck.rules")
+b2sums=('SKIP'
+        'SKIP'
+        'SKIP')
+
+pkgver() {
+  cd "${srcdir}/${pkgname}"
+  git describe --always --tags 2>/dev/null || echo "r0.$(git rev-parse --short HEAD)"
+}
+
+build() {
+  cd "${srcdir}/${pkgname}"
+  export CGO_ENABLED=1
+  go build \
+    -ldflags "-s -w" \
+    -o streamdeck-lets-go ./cmd/streamdeck-lets-go
+}
+
+package() {
+  cd "${srcdir}/${pkgname}"
+
+  install -Dm755 streamdeck-lets-go "${pkgdir}/usr/bin/streamdeck-lets-go"
+
+  install -Dm644 "${srcdir}/packaging/streamdeck-lets-go.service" \
+    "${pkgdir}/usr/lib/systemd/user/streamdeck-lets-go.service"
+
+  install -Dm644 "${srcdir}/packaging/90-streamdeck.rules" \
+    "${pkgdir}/usr/lib/udev/rules.d/90-streamdeck.rules"
+}