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.

Dockerfile

@@ -3,12 +3,20 @@
 # Deploy on Coolify (or any Docker host) with host port mappings
 # 443 -> 8443 and 80 -> 8080 and a volume on /app/data; see README.md.
 
+FROM oven/bun:1 AS web
+WORKDIR /src/web
+COPY web/package.json web/bun.lock ./
+RUN bun install --frozen-lockfile
+COPY web ./
+RUN bun run build
+
 FROM golang:1.27-alpine AS build
 WORKDIR /src
 COPY go.mod go.sum ./
 RUN go mod download
 COPY cmd ./cmd
 COPY internal ./internal
+COPY --from=web /src/internal/dashboard/dist ./internal/dashboard/dist
 RUN CGO_ENABLED=0 go build -trimpath -ldflags="-s -w" -o /out/tunneld ./cmd/tunneld
 
 FROM alpine:3.21