Fix [object Object] in Variables button + allow preview of system templates

- Escape {{ }} in Svelte template (use {'{{ }}'} string expression)
- Allow _get() to access system templates (user_id=0) for preview

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

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

@@ -165,6 +165,6 @@ def _response(c: TemplateConfig) -> dict:
 
 async def _get(session: AsyncSession, config_id: int, user_id: int) -> TemplateConfig:
     config = await session.get(TemplateConfig, config_id)
-    if not config or config.user_id != user_id:
+    if not config or (config.user_id != user_id and config.user_id != 0):
         raise HTTPException(status_code=404, detail="Template config not found")
     return config