Rebuild the admin dashboard as a Svelte 5 + TypeScript app

Frontend in web/ (Vite via bun, pure CSS, no component libraries),
built to internal/dashboard/dist and embedded into the binary; the
Dockerfile builds it in a bun stage. Old static dashboard.html removed.

web/svelte.config.js

@@ -0,0 +1,5 @@
+import { vitePreprocess } from "@sveltejs/vite-plugin-svelte";
+
+export default {
+  preprocess: vitePreprocess(),
+};