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/templates/icons/reset.html

@@ -0,0 +1,11 @@
+<svg xmlns="http://www.w3.org/2000/svg"
+     viewBox="0 0 24 24"
+     fill="none"
+     stroke="currentColor"
+     stroke-width="2"
+     stroke-linecap="round"
+     stroke-linejoin="round"
+     class="text-black dark:text-white w-4 h-4">
+    <path d="M3 12a9 9 0 1 0 3-6.7L3 8" />
+    <path d="M3 3v5h5" />
+</svg>