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/lib/api/mcp.ts

@@ -32,18 +32,16 @@ export const mcpApi = {
     return await invoke("validate_mcp_command", { cmd });
   },
 
+  /**
+   * @deprecated 使用 getAllServers() 代替（v3.7.0+）
+   */
   async getConfig(app: AppId = "claude"): Promise<McpConfigResponse> {
     return await invoke("get_mcp_config", { app });
   },
 
-  async importFromClaude(): Promise<number> {
-    return await invoke("import_mcp_from_claude");
-  },
-
-  async importFromCodex(): Promise<number> {
-    return await invoke("import_mcp_from_codex");
-  },
-
+  /**
+   * @deprecated 使用 upsertUnifiedServer() 代替（v3.7.0+）
+   */
   async upsertServerInConfig(
     app: AppId,
     id: string,
@@ -61,6 +59,9 @@ export const mcpApi = {
     return await invoke("upsert_mcp_server_in_config", payload);
   },
 
+  /**
+   * @deprecated 使用 deleteUnifiedServer() 代替（v3.7.0+）
+   */
   async deleteServerInConfig(
     app: AppId,
     id: string,
@@ -76,26 +77,13 @@ export const mcpApi = {
     return await invoke("delete_mcp_server_in_config", payload);
   },
 
+  /**
+   * @deprecated 使用 toggleApp() 代替（v3.7.0+）
+   */
   async setEnabled(app: AppId, id: string, enabled: boolean): Promise<boolean> {
     return await invoke("set_mcp_enabled", { app, id, enabled });
   },
 
-  async syncEnabledToClaude(): Promise<boolean> {
-    return await invoke("sync_enabled_mcp_to_claude");
-  },
-
-  async syncEnabledToCodex(): Promise<boolean> {
-    return await invoke("sync_enabled_mcp_to_codex");
-  },
-
-  async syncEnabledToGemini(): Promise<boolean> {
-    return await invoke("sync_enabled_mcp_to_gemini");
-  },
-
-  async importFromGemini(): Promise<number> {
-    return await invoke("import_mcp_from_gemini");
-  },
-
   // ========================================================================
   // v3.7.0 新增：统一 MCP 管理 API
   // ========================================================================
@@ -131,11 +119,4 @@ export const mcpApi = {
   ): Promise<void> {
     return await invoke("toggle_mcp_app", { serverId, app, enabled });
   },
-
-  /**
-   * 手动同步所有启用的 MCP 服务器到对应的应用
-   */
-  async syncAllServers(): Promise<void> {
-    return await invoke("sync_all_mcp_servers");
-  },
 };