Refactor admin vendors page to use client-side fetching and pagination

Migrates the admin vendors page to use client-side data fetching with pagination, search, and improved loading states. Updates the vendor table to show last login, status, and admin badges. Also, optimizes the broadcast dialog by lazy-loading the ReactMarkdown component for preview rendering.

UPGRADE_NEXT16.md

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+# Next.js 16 Upgrade Guide
+
+## Current Status
+- **Current Version**: Next.js 15.2.6
+- **Target Version**: Next.js 16.1.1
+- **React Version**: 19.0.0 ✅ (Compatible)
+
+## Upgrade Steps
+
+### 1. Backup First
+```bash
+git add .
+git commit -m "Before Next.js 16 upgrade"
+```
+
+### 2. Update Dependencies
+```bash
+npm install next@16 react@19 react-dom@19 eslint-config-next@16
+```
+
+### 3. Update Related Packages
+```bash
+npm install @next/bundle-analyzer@latest
+```
+
+### 4. Test Build
+```bash
+npm run build
+```
+
+### 5. Test Development Server
+```bash
+npm run dev
+```
+
+## Potential Breaking Changes
+
+### 1. Route Handlers
+- Check all API routes in `app/api/` for compatibility
+- Ensure async/await patterns are correct
+
+### 2. Server Components
+- Verify all server components work correctly
+- Check for any client component boundaries
+
+### 3. Image Optimization
+- `next/image` may have minor API changes
+- Check image imports
+
+### 4. Font Loading
+- `next/font` should work the same, but verify
+
+## Performance Improvements Expected
+
+1. **Faster Builds**: 10-20% faster production builds
+2. **Better Caching**: Improved static generation caching
+3. **Smaller Bundles**: Better tree-shaking and code splitting
+4. **Faster Dev Server**: Turbopack improvements
+
+## Rollback Plan
+
+If issues occur:
+```bash
+npm install next@15.2.6 react@19 react-dom@19 eslint-config-next@15.2.3
+```
+
+## Testing Checklist
+
+- [ ] Build completes successfully
+- [ ] Dev server starts without errors
+- [ ] All pages load correctly
+- [ ] API routes work
+- [ ] Images load properly
+- [ ] Fonts load correctly
+- [ ] Admin dashboard works
+- [ ] Order management works
+- [ ] Analytics charts render
+- [ ] Authentication works
+