feat(FN-2115): add heartbeat multiplier and agent interval controls

- Add heartbeatIntervalMultiplier to shared settings schema/types with settings parity coverage
- Apply heartbeat multiplier in engine scheduling logic while preserving explicit per-agent interval behavior
- Add Settings modal and Agents view controls for heartbeat multiplier and per-agent interval overrides, including new styling
- Expand dashboard and engine test coverage for multiplier/heartbeat controls and document the new setting

packages/core/src/__tests__/settings-parity.test.ts

@@ -49,9 +49,14 @@ describe("settings key parity", () => {
     expect(isGlobalSettingsKey("themeMode")).toBe(true);
     expect(isGlobalSettingsKey("maxConcurrent")).toBe(false);
     expect(isProjectSettingsKey("maxConcurrent")).toBe(true);
+    expect(isProjectSettingsKey("heartbeatMultiplier")).toBe(true);
     expect(isProjectSettingsKey("themeMode")).toBe(false);
   });
 
+  it("includes heartbeatMultiplier in project defaults", () => {
+    expect(DEFAULT_PROJECT_SETTINGS.heartbeatMultiplier).toBe(1);
+  });
+
   it("No key appears in both GLOBAL_SETTINGS_KEYS and PROJECT_SETTINGS_KEYS", () => {
     const projectKeySet = new Set(PROJECT_SETTINGS_KEYS as readonly string[]);
     const overlap = (GLOBAL_SETTINGS_KEYS as readonly string[]).filter((key) => projectKeySet.has(key));