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

packages/server/src/notify_bridge_server/database/seeds.py

@@ -238,6 +238,24 @@ async def _seed_default_tracking_configs() -> None:
                 "name": "Default Webhook",
                 "track_webhook_received": True,
             },
+            {
+                "provider_type": "immich",
+                "name": "Default Immich",
+                "track_assets_added": True,
+                "track_assets_removed": True,
+                "track_collection_renamed": True,
+                "track_collection_deleted": True,
+                "track_sharing_changed": False,
+            },
+            {
+                "provider_type": "google_photos",
+                "name": "Default Google Photos",
+                "track_assets_added": True,
+                "track_assets_removed": True,
+                "track_collection_renamed": True,
+                "track_collection_deleted": True,
+                "track_sharing_changed": False,
+            },
             {
                 "provider_type": "nut",
                 "name": "Default NUT",