refactor: move HTML files to src/, use vite HTML entry points

- Move welcome.html to src/
- Use vite HTML inputs instead of static copy for offscreen.html and welcome.html
- Source HTML references .ts, vite transforms to .js
- Update paths in background.ts to src/

extension/vite.config.mts

@@ -54,8 +54,8 @@ export default defineConfig({
     rollupOptions: {
       input: {
         background: resolve(__dirname, 'src/background.ts'),
-        welcome: resolve(__dirname, 'src/welcome.html'),
         offscreen: resolve(__dirname, 'src/offscreen.html'),
+        welcome: resolve(__dirname, 'src/welcome.html'),
       },
       output: {
         entryFileNames: (chunkInfo) => {