fix(FN-1488): add showQuickChatFAB setting to control Quick Chat FAB visibility

- Add showQuickChatFAB boolean setting to ProjectSettings type
- Add useAppSettings hook function for accessing the setting
- Wire QuickChatFAB visibility in App.tsx based on setting
- Hide QuickChatFAB in MobileNavBar when setting is false
- Add Settings UI toggle for the new setting
- Add CSS for hiding QuickChatFAB on mobile when disabled
- Add comprehensive tests for all affected components
- Fix QuickChatFAB onOpenChange test to use controlled mode
- Update settings reference documentation

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

@@ -99,6 +99,7 @@ const PROJECT_KEYS: (keyof ProjectSettings)[] = [
   "reflectionIntervalMs",
   "reflectionAfterTask",
   "reviewHandoffPolicy",
+  "showQuickChatFAB",
 ];
 
 function assertExactKeyCoverage(scopeName: string, actual: readonly string[], expected: readonly string[]): void {