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/package.json

@@ -0,0 +1,18 @@
+{
+  "name": "tunneld-dashboard",
+  "private": true,
+  "version": "0.0.0",
+  "type": "module",
+  "scripts": {
+    "dev": "vite",
+    "build": "vite build",
+    "check": "svelte-check --tsconfig ./tsconfig.json"
+  },
+  "devDependencies": {
+    "@sveltejs/vite-plugin-svelte": "^7.3.1",
+    "svelte": "^5.57.2",
+    "svelte-check": "^4.7.7",
+    "typescript": "~6",
+    "vite": "^8.3.5"
+  }
+}