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/lib/providers/scheduler.ts

@@ -0,0 +1,24 @@
+import type { ProviderDescriptor } from './types';
+
+export const schedulerDescriptor: ProviderDescriptor = {
+	type: 'scheduler',
+	defaultName: 'Scheduler',
+	icon: 'mdiClockOutline',
+	hasUrl: false,
+
+	configFields: [],
+
+	buildConfig() {
+		return { config: {} };
+	},
+
+	hasConfigChanged() {
+		return false;
+	},
+
+	eventFields: [
+		{ key: 'track_scheduled_message', label: 'trackingConfig.scheduledMessage', default: true },
+	],
+
+	collectionMeta: null,
+};