Refactor Gemini rate limiting configuration

- Removed billingEnabled setting and replaced it with rateLimitingEnabled in GeminiAgent.
- Updated enforceRateLimitForModel function to skip rate limiting based on rateLimitingEnabled.
- Adjusted getGeminiConfig to retrieve rateLimitingEnabled from settings.
- Changed settings management to reflect the new rate limiting logic in SettingsDefaultsManager and UI components.
- Updated ContextSettingsModal to toggle rate limiting instead of billing.
- Ensured default settings reflect the new rate limiting behavior for free tier users.

src/shared/SettingsDefaultsManager.ts

@@ -21,7 +21,7 @@ export interface SettingsDefaults {
   CLAUDE_MEM_PROVIDER: string;  // 'claude' | 'gemini'
   CLAUDE_MEM_GEMINI_API_KEY: string;
   CLAUDE_MEM_GEMINI_MODEL: string;  // 'gemini-2.5-flash-lite' | 'gemini-2.5-flash' | 'gemini-3-flash'
-  CLAUDE_MEM_GEMINI_BILLING_ENABLED: string;  // 'true' | 'false' - skip rate limiting if billing enabled
+  CLAUDE_MEM_GEMINI_RATE_LIMITING_ENABLED: string;  // 'true' | 'false' - enable rate limiting for free tier
   // System Configuration
   CLAUDE_MEM_DATA_DIR: string;
   CLAUDE_MEM_LOG_LEVEL: string;
@@ -59,7 +59,7 @@ export class SettingsDefaultsManager {
     CLAUDE_MEM_PROVIDER: 'claude',  // Default to Claude
     CLAUDE_MEM_GEMINI_API_KEY: '',  // Empty by default, can be set via UI or env
     CLAUDE_MEM_GEMINI_MODEL: 'gemini-2.5-flash-lite',  // Default Gemini model (highest free tier RPM)
-    CLAUDE_MEM_GEMINI_BILLING_ENABLED: 'false',  // Rate limiting enabled by default for no-billing users
+    CLAUDE_MEM_GEMINI_RATE_LIMITING_ENABLED: 'true',  // Rate limiting ON by default for free tier users
     // System Configuration
     CLAUDE_MEM_DATA_DIR: join(homedir(), '.claude-mem'),
     CLAUDE_MEM_LOG_LEVEL: 'INFO',