Jinja2 syntax validation with debounced API check

Two-pass validation in preview-raw endpoint:
1. Syntax check (catches {% if %}, unclosed tags)
2. StrictUndefined render (catches {{ asset.a }}, {{ bad_var }})

Frontend shows:
- Red error for syntax errors with line number
- Amber warning for undefined variables
- Error line highlighted in editor

Sample context now uses proper structured data (lists of dicts
for assets/albums) so valid field access like {{ asset.filename }}
renders correctly during preview.

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

frontend/src/lib/i18n/en.json

@@ -360,6 +360,7 @@
 		"expand": "Expand",
 		"collapse": "Collapse",
 		"syntaxError": "Syntax error",
+		"undefinedVar": "Unknown variable",
 		"line": "line"
 	}
 }