Add a dashboard and backup download to the web UI

Metric history (5s to 1h tiers, persisted), uplink status, ping targets,
DHCP leases, and a tar.gz backup of the state directory.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AeFqV9dZrK6nipHkPfyM4F

tests/apply.nix

@@ -196,5 +196,25 @@ pkgs.testers.runNixOSTest {
         target.wait_until_succeeds("test $(systemctl show -p ActiveState --value router-confirm.service) = inactive")
         assert target.succeed("readlink -f /nix/var/nix/profiles/system").strip() == current()
         assert "match the confirmed system" in curl(f"{ui}/")
+
+    with subtest("the dashboard shows the uplink, the LAN and pings from the router"):
+        curl(f"-d csrf={csrf('/advanced')} --data-urlencode 'pingTargets=gateway, 192.168.10.50' {ui}/prefs")
+        # The UI pings from its sandbox (raw socket); the client answers.
+        # (Into a file: grep -q would end curl with SIGPIPE, failing the pipe.)
+        client.wait_until_succeeds(
+            f"curl -sk -b /tmp/jar -o /tmp/dash '{ui}/dashboard?range=1h' && grep -q 'Ping 192.168.10.50</dt><dd>[0-9.]* ms' /tmp/dash",
+            timeout=60)
+        page = curl(f"{ui}/")
+        # No DHCP server on the uplink in this test.
+        for text in ["Not connected", "LAN main (eth2)", "Internet (eth1)", "Latency", "CPU and memory", "/static/dashboard.js"]:
+            assert text in page, f"{text!r} missing:\n{page}"
+        assert "<html" not in curl(f"'{ui}/dashboard?range=7d'")
+        client.succeed("curl -sfk -o /dev/null https://192.168.10.1/static/dashboard.js")
+
+    with subtest("the web UI hands out a backup of the router's state"):
+        curl(f"-sf -d csrf={csrf('/advanced')} -o /tmp/backup.tar.gz {ui}/backup")
+        files = client.succeed("tar -tzf /tmp/backup.tar.gz")
+        for f in ["router/config/settings.json", "router/webui-password", "router/webui/cert.pem", "router/webui/dashboard.json"]:
+            assert f in files.split(), files
   '';
 }