feat: global Docker health indicator and graceful degradation

- GET /api/health endpoint returning Docker connectivity status
- Sidebar shows Docker connection dot (green=connected, red=disconnected)
- Stale scanner returns store-only results when Docker is unavailable
- Polls health every 30s

web/src/routes/login/+page.svelte

@@ -5,6 +5,7 @@
 	import { t } from '$lib/i18n';
 	import { resolvedTheme, applyTheme } from '$lib/stores/theme';
 	import { IconLoader } from '$lib/components/icons';
+	import { setAuthToken, isAuthenticated } from '$lib/auth';
 
 	let username = $state('');
 	let password = $state('');
@@ -25,7 +26,7 @@
 					headers: { 'Authorization': `Bearer ${urlToken}` }
 				});
 				if (res.ok) {
-					localStorage.setItem('auth_token', urlToken);
+					setAuthToken(urlToken);
 					// Remove token from URL to prevent leakage via history/referrer.
 					history.replaceState(null, '', '/login');
 					goto('/');
@@ -37,8 +38,7 @@
 			// Remove invalid token from URL.
 			history.replaceState(null, '', '/login');
 		}
-		const existingToken = localStorage.getItem('auth_token');
-		if (existingToken) {
+		if (isAuthenticated()) {
 			goto('/');
 		}
 	});
@@ -57,7 +57,7 @@
 				error = envelope.error ?? $t('login.loginFailed');
 				return;
 			}
-			localStorage.setItem('auth_token', envelope.data.token);
+			setAuthToken(envelope.data.token);
 			goto('/');
 		} catch (err: unknown) {
 			error = err instanceof Error ? err.message : $t('login.networkError');