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/main.go

@@ -10,9 +10,11 @@ import (
 	"sync"
 	"time"
 
+	"github.com/adrg/xdg"
 	"golang.org/x/crypto/acme/autocert"
 	"lol.mleku.dev/chk"
 	"lol.mleku.dev/log"
+	"next.orly.dev/app/branding"
 	"next.orly.dev/app/config"
 	"next.orly.dev/pkg/acl"
 	"git.mleku.dev/mleku/nostr/crypto/keys"
@@ -91,6 +93,21 @@ func Run(
 		db:          db,
 	}
 
+	// Initialize branding/white-label manager if enabled
+	if cfg.BrandingEnabled {
+		brandingDir := cfg.BrandingDir
+		if brandingDir == "" {
+			brandingDir = filepath.Join(xdg.ConfigHome, cfg.AppName, "branding")
+		}
+		if _, err := os.Stat(brandingDir); err == nil {
+			if l.brandingMgr, err = branding.New(brandingDir); err != nil {
+				log.W.F("failed to load branding from %s: %v", brandingDir, err)
+			} else {
+				log.I.F("custom branding loaded from %s", brandingDir)
+			}
+		}
+	}
+
 	// Initialize NIP-43 invite manager if enabled
 	if cfg.NIP43Enabled {
 		l.InviteManager = nip43.NewInviteManager(cfg.NIP43InviteExpiry)