refactor(mcp): complete v3.7.0 cleanup - remove legacy code and warnings

This commit finalizes the v3.7.0 unified MCP architecture migration by
removing all deprecated code paths and eliminating compiler warnings.

Frontend Changes (~950 lines removed):
- Remove deprecated components: McpPanel, McpListItem, McpToggle
- Remove deprecated hook: useMcpActions
- Remove unused API methods: importFrom*, syncEnabledTo*, syncAllServers
- Simplify McpFormModal by removing dual-mode logic (unified/legacy)
- Remove syncOtherSide checkbox and conflict detection
- Clean up unused imports and state variables
- Delete associated test files

Backend Changes (~400 lines cleaned):
- Remove unused Tauri commands: import_mcp_from_*, sync_enabled_mcp_to_*
- Delete unused Gemini MCP functions: get_mcp_status, upsert/delete_mcp_server
- Add #[allow(deprecated)] to compatibility layer commands
- Add #[allow(dead_code)] to legacy helper functions for future migration
- Simplify boolean expression in mcp.rs per Clippy suggestion

API Deprecation:
- Mark legacy APIs with @deprecated JSDoc (getConfig, upsertServerInConfig, etc.)
- Preserve backward compatibility for v3.x, planned removal in v4.0

Verification:
- ✅ Zero TypeScript errors (pnpm typecheck)
- ✅ Zero Clippy warnings (cargo clippy)
- ✅ All code formatted (prettier + cargo fmt)
- ✅ Builds successfully

Total cleanup: ~1,350 lines of code removed/marked
Breaking changes: None (all legacy APIs still functional)

src-tauri/src/lib.rs

@@ -6,8 +6,8 @@ mod codex_config;
 mod commands;
 mod config;
 mod error;
-mod gemini_mcp;
 mod gemini_config; // 新增
+mod gemini_mcp;
 mod init_status;
 mod mcp;
 mod prompt;
@@ -541,18 +541,11 @@ pub fn run() {
             commands::upsert_mcp_server_in_config,
             commands::delete_mcp_server_in_config,
             commands::set_mcp_enabled,
-            commands::sync_enabled_mcp_to_claude,
-            commands::sync_enabled_mcp_to_codex,
-            commands::sync_enabled_mcp_to_gemini,
-            commands::import_mcp_from_claude,
-            commands::import_mcp_from_codex,
-            commands::import_mcp_from_gemini,
             // v3.7.0: Unified MCP management
             commands::get_mcp_servers,
             commands::upsert_mcp_server,
             commands::delete_mcp_server,
             commands::toggle_mcp_app,
-            commands::sync_all_mcp_servers,
             // Prompt management
             commands::get_prompts,
             commands::upsert_prompt,