security: deep audit fixes — error leak prevention, DOMPurify XSS guard, mutex race fix, i18n parity, HMAC warning

- [C-1] Fix sync.Mutex copy in guard.GuardStats (go vet race condition)
- [C-2] Replace 3x raw err.Error() HTTP leaks with generic messages (tenant_handlers, ws_transport, immune)
- [M-1] Add isomorphic-dompurify to LegalPage and AIAssistant (XSS defense-in-depth)
- [M-4] Add swaggo/swag dependency for Swagger docs
- [L-4] Add slog.Warn for hardcoded dev HMAC key in tpmaudit
- [L-5] Add 2 missing i18n keys (nav.contacts, nav.start_free) — 365/365 parity

internal/transport/p2p/ws_transport.go

@@ -133,7 +133,8 @@ func (t *WSTransport) handleP2P(w http.ResponseWriter, r *http.Request) {
 		t.mu.RUnlock()
 		if handler != nil {
 			if err := handler(payload); err != nil {
-				http.Error(w, err.Error(), http.StatusInternalServerError)
+				log.Printf("ws-transport: sync handler error: %v", err)
+				http.Error(w, "sync processing failed", http.StatusInternalServerError)
 				return
 			}
 		}