refactor: provider descriptor registry — eliminate provider-specific hardcoding

Replace all if/else chains keyed on provider type strings with a
descriptor-driven architecture. Each provider type (immich, gitea,
planka, scheduler, nut, google_photos) has a descriptor in
frontend/src/lib/providers/ that declares config fields, event
tracking fields, collection metadata, validation, and hooks.

Components now use getDescriptor(type) and render dynamically.
Dashboard provider card shows provider name + type when global
filter is active. Grid-items derived from registry.

frontend/src/routes/notification-trackers/+page.svelte

@@ -16,6 +16,7 @@
 	import { highlightFromUrl } from '$lib/highlight';
 	import { providerDefaultIcon } from '$lib/grid-items';
 	import { globalProviderFilter } from '$lib/stores/provider-filter.svelte';
+	import { getDescriptor } from '$lib/providers';
 	import type { Tracker, TrackerTarget, TrackingConfig, TemplateConfig, NotificationTarget } from '$lib/types';
 
 	import TrackerForm from './TrackerForm.svelte';
@@ -153,33 +154,22 @@
 		e.preventDefault(); error = '';
 		if (submitting) return;
 
-		const newAlbumIds = form.collection_ids.filter(id => !previousCollectionIds.includes(id));
-		if (newAlbumIds.length > 0 && form.provider_id && selectedProviderType === 'immich') {
+		// Delegate provider-specific pre-save checks to the descriptor
+		const desc = getDescriptor(selectedProviderType);
+		if (desc?.onBeforeSave && form.provider_id) {
 			linkCheckLoading = true;
 			try {
-				const missingAlbums: { id: string; name: string; issue: string }[] = [];
-				for (const albumId of newAlbumIds) {
-					interface SharedLink { is_accessible: boolean; is_expired: boolean; has_password: boolean }
-					const links = await api<SharedLink[]>(`/providers/${form.provider_id}/albums/${albumId}/shared-links`);
-					const validLink = links.find((l) => l.is_accessible && !l.is_expired);
-					if (!validLink) {
-						const album = collections.find(c => c.id === albumId);
-						const problematicLink = links.find((l) => l.is_expired || l.has_password);
-						missingAlbums.push({
-							id: albumId,
-							name: album?.albumName || album?.name || albumId,
-							issue: problematicLink
-								? (problematicLink.is_expired ? 'expired' : 'password-protected')
-								: 'missing',
-						});
+				const result = await desc.onBeforeSave({
+					form, previousCollectionIds, collections, api,
+				});
+				if (!result.proceed) {
+					if (result.warnings?.length) {
+						linkWarning = { albums: result.warnings, providerId: form.provider_id };
 					}
-				}
-				if (missingAlbums.length > 0) {
-					linkWarning = { albums: missingAlbums, providerId: form.provider_id };
 					linkCheckLoading = false;
 					return;
 				}
-			} catch (e) { console.warn('Shared link check failed, proceeding:', e); }
+			} catch (err) { console.warn('Pre-save check failed, proceeding:', err); }
 			linkCheckLoading = false;
 		}
 
@@ -277,15 +267,10 @@
 		return p?.name || `#${id}`;
 	}
 
-	const collectionCountLabel: Record<string, string> = {
-		immich: 'notificationTracker.albums_count',
-		gitea: 'notificationTracker.repos_count',
-		planka: 'notificationTracker.boards_count',
-		nut: 'notificationTracker.devices_count',
-	};
 	function getCollectionLabel(tracker: Tracker): string {
 		const pt = getProviderType(tracker);
-		return t(collectionCountLabel[pt] || 'notificationTracker.albums_count');
+		const desc = getDescriptor(pt);
+		return desc?.collectionMeta ? t(desc.collectionMeta.countLabel) : t('notificationTracker.collections_count');
 	}
 
 	function configsForTracker(tracker: Tracker, configs: (TrackingConfig | TemplateConfig)[]): (TrackingConfig | TemplateConfig)[] {