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/handle-relayinfo.go

@@ -115,6 +115,20 @@ func (s *Server) HandleRelayInfo(w http.ResponseWriter, r *http.Request) {
 	description := version.Description + " dashboard: " + s.DashboardURL(r)
 	icon := "https://i.nostr.build/6wGXAn7Zaw9mHxFg.png"
 
+	// Override with branding config if available
+	if s.brandingMgr != nil {
+		nip11 := s.brandingMgr.NIP11Config()
+		if nip11.Name != "" {
+			name = nip11.Name
+		}
+		if nip11.Description != "" {
+			description = nip11.Description
+		}
+		if nip11.Icon != "" {
+			icon = nip11.Icon
+		}
+	}
+
 	// Override with managed ACL config if in managed mode
 	if s.Config.ACLMode == "managed" {
 		// Get managed ACL instance