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/api/template_configs.py

@@ -134,10 +134,10 @@ async def get_template_variables(
         "has_oversized_videos": "Whether any video exceeds the target's size limit (boolean)",
         "max_video_size": "Target video size limit in bytes (null if no limit)",
         "max_video_size_mb": "Target video size limit in MB (null if no limit)",
-        # Immich aliases
+        # Provider-specific aliases
         "album_name": "Alias for collection_name",
         "album_id": "Alias for collection_id",
-        "album_url": "Alias for collection_url",
+        "album_url": "Alias for collection_url (Immich) or album product URL (Google Photos)",
     }
     rename_vars = {
         **event_vars,
@@ -236,7 +236,7 @@ async def get_template_variables(
         "message_scheduled_message": {
             "description": "Notification for scheduled message events",
             "variables": {
-                "tracker_name": "Name of the tracker that fired",
+                "schedule_name": "Name of the schedule that fired",
                 "fire_count": "How many times this tracker has fired",
                 "current_date": "Current date (formatted)",
                 "current_time": "Current time (formatted)",
@@ -420,6 +420,8 @@ async def update_config(
     session: AsyncSession = Depends(get_session),
 ):
     config = await _get(session, config_id, user.id)
+    if config.user_id == 0 and user.role != "admin":
+        raise HTTPException(status_code=403, detail="Cannot modify system default configs")
     for field, value in body.model_dump(exclude_unset=True, exclude={"slots"}).items():
         if value is not None:
             setattr(config, field, value)