refactor: comprehensive consistency review — UI/UX, code quality, functional parity

Fix 19 issues across 3 priority tiers found during full-codebase review:

CRITICAL:
- Fix undefined --color-secondary CSS variable causing invisible UI elements
- Fix Google Photos command templates using nonexistent asset.originalFileName
- Fix scheduler template variable docs (tracker_name → schedule_name)
- Add missing admin guard on notification template update endpoint

HIGH:
- Fix 5 hardcoded English strings missing i18n (MultiEntitySelect, actions,
  settings, TelegramBotTab, users)
- Replace 17 raw <button> elements with shared <Button> component
- Replace 5 raw error divs with shared <ErrorBanner> component
- Refactor webhook handler duplication into shared _dispatch_webhook_event()
- Add 30+ provider-specific fields to TrackingConfig TypeScript type
- Add default TrackingConfig seeds for immich and google_photos
- Add provider-specific command variable docs (Gitea, Planka, NUT, GP, Webhook)

MEDIUM:
- Replace hardcoded hex colors and Tailwind classes with CSS variable tokens
- Remove dead code (unused imports, orphaned check_notification_tracker)
- Fix Svelte 5 patterns ($state for _prevProviderId, remove unnecessary as any)
- Fix inconsistent POST response shape (targets now returns full response)
- Fix Google Photos template dead asset.year branches, clarify album_url docs

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

@@ -131,7 +131,7 @@
 		try { collections = await api(`/providers/${form.provider_id}/collections`); } catch (e) { console.warn('Failed to load collections:', e); collections = []; }
 	}
 
-	let _prevProviderId = 0;
+	let _prevProviderId = $state(0);
 	$effect(() => {
 		if (showForm && form.provider_id && form.provider_id !== _prevProviderId) {
 			_prevProviderId = form.provider_id;
@@ -146,8 +146,8 @@
 			name: trk.name, icon: trk.icon || '', provider_id: trk.provider_id,
 			collection_ids: [...(trk.collection_ids || [])],
 			scan_interval: trk.scan_interval, batch_duration: trk.batch_duration ?? 0,
-			default_tracking_config_id: (trk as any).default_tracking_config_id || 0,
-			default_template_config_id: (trk as any).default_template_config_id || 0,
+			default_tracking_config_id: trk.default_tracking_config_id ?? 0,
+			default_template_config_id: trk.default_template_config_id ?? 0,
 			filters: trk.filters || {},
 		};
 		previousCollectionIds = [...(trk.collection_ids || [])];