feat(branding): add white-label branding system (v0.52.0)

Add runtime-customizable branding allowing relay operators to fully
customize UI appearance without rebuilding:

- Custom logo, favicon, and PWA icons
- Full CSS override capability (colors, themes, components)
- Custom app name, title, and NIP-11 relay info
- init-branding command with --style generic|orly options
- Transparent PNG generation for generic branding

New files:
- app/branding/ package (branding.go, init.go, types.go)
- docs/BRANDING_GUIDE.md

Environment variables:
- ORLY_BRANDING_DIR: branding directory path
- ORLY_BRANDING_ENABLED: enable/disable custom branding

Usage: ./orly init-branding --style generic

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/web.go

@@ -23,3 +23,9 @@ func ServeEmbeddedWeb(w http.ResponseWriter, r *http.Request) {
 	// Serve the embedded web app
 	http.FileServer(GetReactAppFS()).ServeHTTP(w, r)
 }
+
+// GetEmbeddedWebFS returns the raw embedded filesystem for branding initialization.
+// This is used by the init-branding command to extract default assets.
+func GetEmbeddedWebFS() embed.FS {
+	return reactAppFS
+}