feat(settings): harden settings API and overhaul SettingsModal

Security:
- Strip auth credential keys (auth_username, auth_password_hash,
  auth_jwt_secret) from GET /api/settings response
- Add allowlist guard to POST /api/settings — rejects unknown or
  auth-namespace keys with a 400

Backend:
- Add PATCH /api/settings bulk endpoint with Zod schema validation
  (type coercion, range checks, URL format) and atomic SQLite transaction
- Add system_state table — moves last_janitor_alert_timestamp out of
  global_settings; adds getSystemState/setSystemState on DatabaseService
- Add metrics_retention_hours and log_retention_days configurable settings;
  MonitorService reads both dynamically each evaluation cycle
- Add cleanupOldNotifications(days) to DatabaseService, called each cycle

Frontend:
- Replace single isLoading flag with per-operation states
  (isSavingSystem, isSavingDeveloper, isSavingPassword, isSavingRegistry,
  isSavingAgent/isTestingAgent per agent type)
- Add skeleton loader that blocks interaction until fetchSettings resolves
- Explicit key-picking in fetchSettings — auth keys cannot enter state
- Unsaved-changes amber dot on System Limits and Developer sidebar items
- Separate saveSystemSettings / saveDeveloperSettings — no cross-tab clobber
- Developer tab gains Data Retention section (metrics hours, log days)
- All settings saves use new PATCH /api/settings endpoint

backend/package.json

@@ -45,6 +45,7 @@
     "node-pty": "^1.1.0",
     "systeminformation": "^5.31.1",
     "ws": "^8.19.0",
-    "yaml": "^2.8.2"
+    "yaml": "^2.8.2",
+    "zod": "^4.3.6"
   }
 }