Polish: legible crew roster, End-Scene enforcement, decluttered Hand

- Crew roster bubbles: explicit text color + opaque distinct surface so
  they're legible and stand out from the page in both themes (the bug was
  the <button> defaulting to black-on-dark in dark mode).
- End-Scene enforcement: unless Dev Mode is on, a scene can't end until
  every living Pi-Rat has faced a Deep Challenge or the Obstacle List is
  empty. end_scene_and_transition now returns (ok, msg) and the route
  surfaces a 400; the Deep sees a ✓/○ challenge-progress badge on each
  Pi-Rat bubble. 3 new tests cover the gate.
- Hand panel: dropped the how-to-play blurb and the title in favor of a
  low-profile "Your Hand" label.
- Bump VERSION to 8 with changelog entry; check off TODO.md Polish items.

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

frontend/src/components/ScenePhase.svelte

@@ -27,8 +27,7 @@
     <div class="table-column">
         {#if !isDeep}
             <div class="card glass-panel hand-card">
-                <h3>🃏 Your Hand</h3>
-                <p class="section-desc">Keep your cards secret! When the Deep challenges you (or you assist a crewmate), drag a card onto an applied obstacle to play it. Jokers can hit any obstacle, any time.</p>
+                <span class="hand-label">🃏 Your Hand</span>
 
                 <div class="hand-flex">
                     {#each hand as card}