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- Clean up `src/vite-env.d.ts` (remove 156 lines of outdated types)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/providers/ProviderList.tsx

@@ -1,8 +1,5 @@
 import { CSS } from "@dnd-kit/utilities";
-import {
-  DndContext,
-  closestCenter,
-} from "@dnd-kit/core";
+import { DndContext, closestCenter } from "@dnd-kit/core";
 import {
   SortableContext,
   useSortable,
@@ -137,9 +134,7 @@ function SortableProviderCard({
         onEdit={onEdit}
         onDelete={onDelete}
         onConfigureUsage={
-          onConfigureUsage
-            ? (item) => onConfigureUsage(item)
-            : () => undefined
+          onConfigureUsage ? (item) => onConfigureUsage(item) : () => undefined
         }
         onOpenWebsite={onOpenWebsite}
         dragHandleProps={{