fix: add nl2br filter and fix TemplateResponse arguments

- Add nl2br Jinja2 filter to convert newlines to <br> tags
- Fix TemplateResponse argument order (request first) in error handlers
- Fix type annotations for mypy
- All 97 tests passing

app/presentation/web/routes.py

@@ -28,6 +28,21 @@ router = APIRouter(prefix="/web", tags=["web"])
 templates = Jinja2Templates(directory="app/presentation/templates")
 
 
+def nl2br(value: str) -> str:
+    """Convert newlines to HTML line breaks.
+
+    Args:
+        value: String with newlines.
+
+    Returns:
+        String with <br> tags instead of newlines.
+    """
+    return value.replace("\n", "<br>\n")
+
+
+templates.env.filters["nl2br"] = nl2br
+
+
 class MockPost:
     """Mock post object for UI demonstration.