Jinja2 syntax highlighting + description field + preview toggle

- Error line highlighting in JinjaEditor (red background on error line)
- Backend returns error_line from TemplateSyntaxError
- Localized syntax error messages with line number
- Renamed {{ }} button to "Variables" (localized)
- Localized all template variable descriptions (EN/RU)
- Added t() fallback parameter for graceful degradation
- Page transition animation (fade) to prevent content stacking
- Added syntaxError/line i18n keys

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/lib/i18n/en.json

@@ -266,8 +266,48 @@
 		"telegramSettings": "Telegram",
 		"videoWarning": "Video warning",
 		"preview": "Preview",
+		"variables": "Variables",
+		"assetFields": "Asset fields (in {% for asset in added_assets %})",
 		"confirmDelete": "Delete this template config?"
 	},
+	"templateVars": {
+		"message_assets_added": { "description": "Notification when new assets are added to an album" },
+		"message_assets_removed": { "description": "Notification when assets are removed from an album" },
+		"message_album_renamed": { "description": "Notification when an album is renamed" },
+		"message_album_deleted": { "description": "Notification when an album is deleted" },
+		"periodic_summary_message": { "description": "Periodic album summary with stats" },
+		"scheduled_assets_message": { "description": "Scheduled asset picks from albums" },
+		"memory_mode_message": { "description": "\"On This Day\" memories from past years" },
+		"album_name": "Album name",
+		"album_url": "Public share URL (if available)",
+		"added_count": "Number of assets added",
+		"removed_count": "Number of assets removed",
+		"change_type": "Type of change",
+		"people": "Detected people names (use {{ people | join(', ') }})",
+		"added_assets": "List of added asset objects (use {% for asset in added_assets %})",
+		"removed_assets": "List of removed asset IDs",
+		"shared": "Whether album is shared (true/false)",
+		"video_warning": "Video size warning text",
+		"old_name": "Previous album name",
+		"new_name": "New album name",
+		"albums": "List of album objects (use {% for album in albums %})",
+		"assets": "List of asset objects (use {% for asset in assets %})",
+		"date": "Current date/time",
+		"asset_filename": "Original filename",
+		"asset_type": "IMAGE or VIDEO",
+		"asset_created_at": "Creation date/time (ISO 8601)",
+		"asset_owner": "Owner display name",
+		"asset_description": "User or EXIF description",
+		"asset_url": "Public viewer URL",
+		"asset_download_url": "Direct download URL",
+		"asset_photo_url": "Preview image URL (images only)",
+		"asset_playback_url": "Video playback URL (videos only)",
+		"asset_is_favorite": "Whether asset is favorited",
+		"asset_rating": "Star rating (1-5 or null)",
+		"asset_city": "City name",
+		"asset_state": "State/region name",
+		"asset_country": "Country name"
+	},
 	"hints": {
 		"periodicSummary": "Sends a scheduled summary of all tracked albums at specified times. Great for daily/weekly digests.",
 		"scheduledAssets": "Sends random or selected photos from tracked albums on a schedule. Like a daily photo pick.",
@@ -318,6 +358,8 @@
 		"newPassword": "New password",
 		"passwordChanged": "Password changed successfully",
 		"expand": "Expand",
-		"collapse": "Collapse"
+		"collapse": "Collapse",
+		"syntaxError": "Syntax error",
+		"line": "line"
 	}
 }