feat(FN-1627): expose global execution concurrency limit in settings

- Add global max concurrent agent limit to settings UI (Scheduling section)
- Add null-as-delete semantics for global settings persistence
- Add fetchGlobalConcurrency and updateGlobalConcurrency API integrations
- Update SettingsModal tests with comprehensive coverage for new features
- Fix lint issues in modified files

packages/dashboard/app/components/SettingsModal.test.tsx

@@ -39,8 +39,6 @@ vi.mock("../api", () => ({
   saveMemory: (...args: unknown[]) => mockSaveMemory(...args),
   fetchGlobalConcurrency: (...args: unknown[]) => mockFetchGlobalConcurrency(...args),
   updateGlobalConcurrency: (...args: unknown[]) => mockUpdateGlobalConcurrency(...args),
-  saveApiKey: vi.fn().mockResolvedValue(undefined),
-  clearApiKey: vi.fn().mockResolvedValue(undefined),
 }));
 
 const noop = () => {};