feat(modals): layout context processor for HX-Modal header

Switches the Jinja base layout to _modal_partial.html (yield-only) when
the HX-Modal:1 request header is set, otherwise base.html. Foundation
for URL-addressable modals (spec 2026-05-17-url-addressable-modals).

Guards with has_request_context() so the processor is safe when
render_template_string is called from app_context() without a request
(e.g. test_timeago_filter_registered_on_app).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

l4d2web/l4d2web/app.py

@@ -61,6 +61,13 @@ def create_app(test_config: dict[str, object] | None = None) -> Flask:
 
     app.add_template_filter(format_time_html, "timeago")
 
+    @app.context_processor
+    def inject_base_layout() -> dict[str, str]:
+        from flask import has_request_context
+
+        is_modal = has_request_context() and request.headers.get("HX-Modal") == "1"
+        return {"base_layout": "_modal_partial.html" if is_modal else "base.html"}
+
     @app.before_request
     def csrf_protect() -> Response | None:
         if "csrf_token" not in session:

l4d2web/l4d2web/templates/_modal_partial.html

@@ -0,0 +1 @@
+{% block content %}{% endblock %}

l4d2web/tests/test_url_addressable_modals.py

@@ -0,0 +1,29 @@
+from flask import render_template_string
+
+from l4d2web.app import create_app
+
+
+def _make_app(tmp_path, monkeypatch, db_name: str):
+    db_url = f"sqlite:///{tmp_path/db_name}"
+    monkeypatch.setenv("DATABASE_URL", db_url)
+    return create_app({"TESTING": True, "DATABASE_URL": db_url, "SECRET_KEY": "test"})
+
+
+def test_base_layout_is_modal_partial_when_hx_modal_header_set(tmp_path, monkeypatch):
+    app = _make_app(tmp_path, monkeypatch, "layout-modal.db")
+    with app.test_request_context("/", headers={"HX-Modal": "1"}):
+        assert render_template_string("{{ base_layout }}") == "_modal_partial.html"
+
+
+def test_base_layout_is_base_html_for_normal_request(tmp_path, monkeypatch):
+    app = _make_app(tmp_path, monkeypatch, "layout-default.db")
+    with app.test_request_context("/"):
+        assert render_template_string("{{ base_layout }}") == "base.html"
+
+
+def test_base_layout_does_not_react_to_plain_hx_request_header(tmp_path, monkeypatch):
+    # HTMX sets HX-Request on every request including the build-status poll;
+    # only HX-Modal should switch the layout.
+    app = _make_app(tmp_path, monkeypatch, "layout-hxreq.db")
+    with app.test_request_context("/", headers={"HX-Request": "true"}):
+        assert render_template_string("{{ base_layout }}") == "base.html"