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.

.gitignore

@@ -2,3 +2,6 @@
 cmd/tunneld/tunneld
 .local/
 .DS_Store
+web/node_modules/
+internal/dashboard/dist/*
+!internal/dashboard/dist/.gitkeep