fix(i18n): register _() Jinja2 global and current_locale in error handlers

Error handlers had a separate Jinja2Templates instance without the _
global function, causing UndefinedError when rendering base.html
(which now calls {{ _(key, current_locale) }}).

- Register _() from translator module as Jinja2 global on error_handlers templates
- Add current_locale to get_template_context() from request.state.locale

app/presentation/web/error_handlers.py

@@ -10,9 +10,11 @@ from fastapi import HTTPException, Request
 from fastapi.responses import HTMLResponse, RedirectResponse
 from fastapi.templating import Jinja2Templates
 
+from app.infrastructure.i18n.translator import DEFAULT_LOCALE, _
 from app.presentation.web.flash import FlashManager, get_flash_messages
 
 templates = Jinja2Templates(directory="app/presentation/templates")
+templates.env.globals["_"] = _
 
 
 async def setup_flash_manager(request: Request) -> None:
@@ -55,6 +57,7 @@ def get_template_context(request: Request) -> dict[str, Any]:
         "user_role": user_role.value if user_role else None,
         "can_create": can_create_post(user),
         "flash_messages": get_flash_messages(request),
+        "current_locale": getattr(request.state, "locale", DEFAULT_LOCALE),
     }