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/claude_mcp.rs

@@ -242,11 +242,7 @@ pub fn read_mcp_servers_map() -> Result<std::collections::HashMap<String, Value>
     let servers = root
         .get("mcpServers")
         .and_then(|v| v.as_object())
-        .map(|obj| {
-            obj.iter()
-                .map(|(k, v)| (k.clone(), v.clone()))
-                .collect()
-        })
+        .map(|obj| obj.iter().map(|(k, v)| (k.clone(), v.clone())).collect())
         .unwrap_or_default();
 
     Ok(servers)