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>

packages/server/src/immich_watcher_server/api/template_configs.py

@@ -6,6 +6,7 @@ from sqlmodel import select
 from sqlmodel.ext.asyncio.session import AsyncSession
 
 from jinja2.sandbox import SandboxedEnvironment
+from jinja2 import TemplateSyntaxError, UndefinedError
 
 from ..auth.dependencies import get_current_user
 from ..database.engine import get_session
@@ -172,8 +173,16 @@ async def preview_raw(
         tmpl = env.from_string(body.template)
         rendered = tmpl.render(**_SAMPLE_CONTEXT)
         return {"rendered": rendered}
+    except TemplateSyntaxError as e:
+        return {
+            "rendered": None,
+            "error": e.message,
+            "error_line": e.lineno,
+        }
+    except UndefinedError as e:
+        return {"rendered": None, "error": str(e), "error_line": None}
     except Exception as e:
-        return {"rendered": None, "error": str(e)}
+        return {"rendered": None, "error": str(e), "error_line": None}
 
 
 def _response(c: TemplateConfig) -> dict: