feat(gemini): add config.json editor and common config functionality

Implements dual-editor pattern for Gemini providers, following the Codex architecture:
- Environment variables (.env format) editor
- Extended configuration (config.json) editor with common config support

New Components:
- GeminiConfigSections: Separate sections for env and config editing
- GeminiCommonConfigModal: Modal for editing common config snippets

New Hooks:
- useGeminiConfigState: Manages env/config separation and conversion
  - Converts between .env string format and JSON object
  - Validates JSON config structure
  - Extracts API Key and Base URL from env
- useGeminiCommonConfig: Handles common config snippets
  - Deep merge algorithm for combining configs
  - Remove common config logic for toggling off
  - localStorage persistence for snippets

Features:
- Format buttons for both env and config editors
- Common config toggle with deep merge/remove
- Error validation and display
- Auto-open modal on common config errors

Configuration Structure:
{
  "env": {
    "GOOGLE_GEMINI_BASE_URL": "https://...",
    "GEMINI_API_KEY": "sk-...",
    "GEMINI_MODEL": "gemini-2.5-pro"
  },
  "config": {
    "timeout": 30000,
    "maxRetries": 3
  }
}

This brings Gemini providers to feature parity with Claude and Codex.

src/components/providers/forms/hooks/index.ts

@@ -10,3 +10,5 @@ export { useCommonConfigSnippet } from "./useCommonConfigSnippet";
 export { useCodexCommonConfig } from "./useCodexCommonConfig";
 export { useSpeedTestEndpoints } from "./useSpeedTestEndpoints";
 export { useCodexTomlValidation } from "./useCodexTomlValidation";
+export { useGeminiConfigState } from "./useGeminiConfigState";
+export { useGeminiCommonConfig } from "./useGeminiCommonConfig";