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

pkgs/router-webui/main_test.go

@@ -19,7 +19,7 @@ func newTestApp(t *testing.T) (*app, System) {
 	os.WriteFile(sys.settingsPath(), []byte(sample), 0o600)
 	h, _ := hashPassword("hunter2hunter2")
 	os.WriteFile(sys.passwordPath(), []byte(h), 0o600)
-	return newApp(sys), sys
+	return newApp(sys, newCollector(sys, filepath.Join(dir, "leases")), false), sys
 }
 
 func do(h http.Handler, method, path string, form url.Values, cookie *http.Cookie) *httptest.ResponseRecorder {