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/components/ui/checkbox.tsx

@@ -1,8 +1,8 @@
-import * as React from "react"
-import * as CheckboxPrimitive from "@radix-ui/react-checkbox"
-import { Check } from "lucide-react"
+import * as React from "react";
+import * as CheckboxPrimitive from "@radix-ui/react-checkbox";
+import { Check } from "lucide-react";
 
-import { cn } from "@/lib/utils"
+import { cn } from "@/lib/utils";
 
 const Checkbox = React.forwardRef<
   React.ElementRef<typeof CheckboxPrimitive.Root>,
@@ -12,7 +12,7 @@ const Checkbox = React.forwardRef<
     ref={ref}
     className={cn(
       "grid place-content-center peer h-4 w-4 shrink-0 rounded-sm border border-primary ring-offset-background focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-ring focus-visible:ring-offset-2 disabled:cursor-not-allowed disabled:opacity-50 data-[state=checked]:bg-primary data-[state=checked]:text-primary-foreground",
-      className
+      className,
     )}
     {...props}
   >
@@ -22,7 +22,7 @@ const Checkbox = React.forwardRef<
       <Check className="h-4 w-4" />
     </CheckboxPrimitive.Indicator>
   </CheckboxPrimitive.Root>
-))
-Checkbox.displayName = CheckboxPrimitive.Root.displayName
+));
+Checkbox.displayName = CheckboxPrimitive.Root.displayName;
 
-export { Checkbox }
+export { Checkbox };