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/src/lib/SummaryCards.svelte

@@ -0,0 +1,28 @@
+<script lang="ts">
+  import type { CaInfo, ServerInfo, Totals } from "./api";
+  import { fmtDate, fmtUptime } from "./format";
+
+  let { server, ca, totals }: { server: ServerInfo; ca: CaInfo; totals: Totals } = $props();
+</script>
+
+<div class="cards">
+  <div class="card"><div class="label">Tunnels</div><div class="value">{totals.tunnels}</div></div>
+  <div class="card"><div class="label">Online</div><div class="value">{totals.online}</div></div>
+  <div class="card"><div class="label">Bridges</div><div class="value">{totals.bridges}</div></div>
+  <div class="card"><div class="label">Visitors</div><div class="value">{totals.visitors}</div></div>
+  <div class="card"><div class="label">CA expires</div><div class="value small">{fmtDate(ca.expires)}</div></div>
+  <div class="card"><div class="label">Uptime</div><div class="value small">{fmtUptime(server.uptime_seconds)}</div></div>
+</div>
+
+<style>
+  .cards {
+    display: grid;
+    grid-template-columns: repeat(auto-fit, minmax(140px, 1fr));
+    gap: 12px;
+    margin-bottom: 24px;
+  }
+  .value.small {
+    font-size: 14px;
+    margin-top: 8px;
+  }
+</style>