feat(session): reset running session start time to now from list

Adds a confirm-gated button on running sessions in the session list that
sets timestamp_start to now (issue #33). The htmx path returns HX-Refresh;
ButtonGroup gains optional hx_confirm/hx_swap keys.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

games/urls.py

@@ -147,6 +147,11 @@ urlpatterns = [
         session.end_session,
         name="list_sessions_end_session",
     ),
+    path(
+        "session/start/reset-to-now/from-list/<int:session_id>",
+        session.reset_session_start,
+        name="list_sessions_reset_session_start",
+    ),
     path("session/list", session.list_sessions, name="list_sessions"),
     path("session/search", session.search_sessions, name="search_sessions"),
     path(