Add version number + About dialog with changelog (v1)

Start tracking a version number and a player-facing changelog, surfaced from
the ☰ menu via a new About item.

- frontend/src/lib/changelog.js: VERSION (bump every commit) + CHANGELOG.
- AboutModal.svelte: shows the version and changelog using the shared modal CSS;
  closes via ×, backdrop click, or Escape.
- CornerMenu.svelte: new 'ℹ️ About' menu item, modal rendered outside the menu's
  stacking context so it overlays correctly on every page.
- AGENTS.md: documents the bump-every-commit / log-only-user-facing convention.

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

frontend/src/lib/changelog.js

@@ -0,0 +1,21 @@
+// App version and player-facing changelog, surfaced in the ☰ menu → About
+// (rendered by AboutModal.svelte).
+//
+// Maintenance (see AGENTS.md "Versioning & changelog"):
+//   - Bump VERSION by one on every commit.
+//   - 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 = 1;
+
+// Newest first. Each entry: { version, date: 'YYYY-MM-DD', changes: [string, ...] }.
+export const CHANGELOG = [
+    {
+        version: 1,
+        date: '2026-06-15',
+        changes: [
+            'First tracked version — the app now reports its version here in the ☰ menu → About, with a log of player-facing changes.',
+            'Home screen lists crews you can rejoin above “Muster a New Crew”.',
+        ],
+    },
+];