feat: complete stage 4 cleanup and code formatting

This commit completes stage 4 of the refactoring plan, focusing on cleanup
and optimization of the modernized codebase.

## Key Changes

### Code Cleanup
- Remove legacy `src/lib/styles.ts` (no longer needed)
- Remove old modal components (`ImportProgressModal.tsx`, `ProviderList.tsx`)
- Streamline `src/lib/tauri-api.ts` from 712 lines to 17 lines (-97.6%)
  - Remove global `window.api` pollution
  - Keep only event listeners (`tauriEvents.onProviderSwitched`)
  - All API calls now use modular `@/lib/api/*` layer

### Type System
- Clean up `src/vite-env.d.ts` (remove 156 lines of outdated types)
- Remove obsolete global type declarations
- All TypeScript checks pass with zero errors

### Code Formatting
- Format all source files with Prettier (82 files)
- Fix formatting issues in 15 files:
  - App.tsx and core components
  - MCP management components
  - Settings module components
  - Provider management components
  - UI components

### Documentation Updates
- Update `REFACTORING_CHECKLIST.md` with stage 4 progress
- Mark completed tasks in `REFACTORING_MASTER_PLAN.md`

## Impact

**Code Reduction:**
- Total: -1,753 lines, +384 lines (net -1,369 lines)
- tauri-api.ts: 712 → 17 lines (-97.6%)
- Removed styles.ts: -82 lines
- Removed vite-env.d.ts declarations: -156 lines

**Quality Improvements:**
- ✅ Zero TypeScript errors
- ✅ Zero TODO/FIXME comments
- ✅ 100% Prettier compliant
- ✅ Zero `window.api` references
- ✅ Fully modular API layer

## Testing
- [x] TypeScript compilation passes
- [x] Code formatting validated
- [x] No linting errors

Stage 4 completion: 100%
Ready for stage 5 (testing and bug fixes)

src/components/ui/select.tsx

@@ -17,7 +17,7 @@ const SelectTrigger = React.forwardRef<
     ref={ref}
     className={cn(
       "flex h-9 w-full items-center justify-between whitespace-nowrap rounded-md border border-input bg-background px-3 py-2 text-sm shadow-sm ring-offset-background placeholder:text-muted-foreground focus:outline-none focus:ring-1 focus:ring-ring disabled:cursor-not-allowed disabled:opacity-50",
-      className
+      className,
     )}
     {...props}
   >
@@ -38,7 +38,7 @@ const SelectContent = React.forwardRef<
       ref={ref}
       className={cn(
         "relative z-50 min-w-[8rem] overflow-hidden rounded-md border bg-popover text-popover-foreground shadow-md data-[state=open]:animate-in data-[state=closed]:animate-out data-[state=closed]:fade-out-0 data-[state=open]:fade-in-0 data-[state=closed]:zoom-out-95 data-[state=open]:zoom-in-95 data-[side=bottom]:slide-in-from-top-2 data-[side=left]:slide-in-from-right-2 data-[side=right]:slide-in-from-left-2 data-[side=top]:slide-in-from-bottom-2",
-        className
+        className,
       )}
       position={position}
       {...props}
@@ -50,7 +50,7 @@ const SelectContent = React.forwardRef<
         className={cn(
           "p-1",
           position === "popper" &&
-            "h-[var(--radix-select-trigger-height)] w-full min-w-[var(--radix-select-trigger-width)]"
+            "h-[var(--radix-select-trigger-height)] w-full min-w-[var(--radix-select-trigger-width)]",
         )}
       >
         {children}
@@ -83,7 +83,7 @@ const SelectItem = React.forwardRef<
     ref={ref}
     className={cn(
       "relative flex w-full cursor-default select-none items-center rounded-sm px-2 py-1.5 text-sm outline-none focus:bg-accent focus:text-accent-foreground data-[disabled]:pointer-events-none data-[disabled]:opacity-50",
-      className
+      className,
     )}
     {...props}
   >