feat(gemini): update Gemini model types and implement rate limiting for free tier

- Changed Gemini model types to 'gemini-2.5-flash-lite', 'gemini-2.5-flash', and 'gemini-3-flash'.
- Introduced RPM limits for free tier models with a maximum of 10 RPM for 'gemini-2.5-flash-lite' and 5 RPM for the others.
- Added rate limiting enforcement in the GeminiAgent class, which waits based on the model's RPM limit.
- Updated getGeminiConfig to include billingEnabled setting, allowing users to skip rate limiting if billing is enabled.
- Modified ContextSettingsModal to reflect new model options and added a toggle for enabling billing.
- Updated default settings to use the new model and billing configuration.

src/ui/viewer/constants/settings.ts

@@ -11,7 +11,8 @@ export const DEFAULT_SETTINGS = {
   // AI Provider Configuration
   CLAUDE_MEM_PROVIDER: 'claude',
   CLAUDE_MEM_GEMINI_API_KEY: '',
-  CLAUDE_MEM_GEMINI_MODEL: 'gemini-2.0-flash-exp',
+  CLAUDE_MEM_GEMINI_MODEL: 'gemini-2.5-flash-lite',
+  CLAUDE_MEM_GEMINI_BILLING_ENABLED: 'false',
 
   // Token Economics (all true for backwards compatibility)
   CLAUDE_MEM_CONTEXT_SHOW_READ_TOKENS: 'true',