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

cmd/immune/main.go

@@ -115,7 +115,8 @@ func main() {
 				Mode string `json:"mode"`
 			}
 			if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
-				http.Error(w, err.Error(), http.StatusBadRequest)
+				slog.Error("invalid mode request", "error", err)
+				http.Error(w, "invalid request body", http.StatusBadRequest)
 				return
 			}
 			g.SetMode(guard.Mode(req.Mode))