feat: configurable unused images threshold with dashboard warning

- Add image_prune_threshold_mb setting (default 1024 MB)
- Add GET /api/docker/unused-images endpoint returning unused image count, size, and threshold status
- Dashboard shows amber warning banner when unused project images exceed threshold
- Banner links to settings page for pruning, shows count and human-readable size (MB/GB)
- Threshold configurable in Docker Image Cleanup section of settings
- DB migration + schema for image_prune_threshold_mb

web/src/routes/+page.svelte

@@ -11,6 +11,9 @@
 	let projects = $state<Project[]>([]);
 	let instancesByProject = $state<Record<string, Instance[]>>({});
 	let staleContainers = $state<StaleContainer[]>([]);
+	let unusedImagesMB = $state(0);
+	let unusedImagesCount = $state(0);
+	let unusedImagesExceeded = $state(false);
 	let loading = $state(true);
 	let error = $state('');
 
@@ -43,6 +46,13 @@
 			}
 			instancesByProject = mapped;
 			staleContainers = staleResult;
+
+			try {
+				const imgStats = await api.getUnusedImageStats();
+				unusedImagesMB = imgStats.total_size_mb;
+				unusedImagesCount = imgStats.count;
+				unusedImagesExceeded = imgStats.exceeded;
+			} catch { /* non-critical */ }
 		} catch (e) {
 			error = e instanceof Error ? e.message : $t('dashboard.loadFailed');
 		} finally {
@@ -125,6 +135,20 @@
 		</a>
 	</div>
 
+	<!-- Unused images warning -->
+	{#if unusedImagesExceeded}
+		<a href="/settings" class="flex items-center gap-3 rounded-xl border border-amber-300 bg-amber-50 dark:border-amber-700 dark:bg-amber-950/30 p-4 transition-colors hover:bg-amber-100 dark:hover:bg-amber-950/50">
+			<div class="flex h-10 w-10 shrink-0 items-center justify-center rounded-lg bg-amber-100 dark:bg-amber-900/50 text-amber-600">
+				<IconAlert size={20} />
+			</div>
+			<div class="flex-1">
+				<p class="text-sm font-medium text-amber-800 dark:text-amber-300">{$t('dashboard.unusedImagesWarning')}</p>
+				<p class="text-xs text-amber-700 dark:text-amber-400">{unusedImagesCount} {$t('dashboard.unusedImages')} &middot; {unusedImagesMB >= 1024 ? (unusedImagesMB / 1024).toFixed(1) + ' GB' : unusedImagesMB + ' MB'}</p>
+			</div>
+			<span class="text-xs font-medium text-amber-600 dark:text-amber-400">{$t('settings.pruneImages')} &rarr;</span>
+		</a>
+	{/if}
+
 	<!-- System health summary -->
 	<SystemHealthCard />