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/lib/changelog.js

@@ -6,10 +6,19 @@
 //   - Add a CHANGELOG entry only when a commit changes something players can
 //     see. Skip refactors, tests, and tooling. Keep wording player-facing.
 
-export const VERSION = 7;
+export const VERSION = 8;
 
 // Newest first. Each entry: { version, date: 'YYYY-MM-DD', changes: [string, ...] }.
 export const CHANGELOG = [
+    {
+        version: 8,
+        date: '2026-06-15',
+        changes: [
+            'Crew roster bubbles are now legible and stand out from the background in both light and dark mode.',
+            'A scene can no longer be ended until every Pi-Rat has faced a Challenge (or the Obstacle List is empty). The Deep sees a ✓/○ marker on each Pi-Rat showing who still needs one.',
+            'Decluttered your Hand panel — dropped the how-to-play blurb for a low-profile label.',
+        ],
+    },
     {
         version: 7,
         date: '2026-06-15',