Add public Shard page + player Game Accounts UI (phase 4)

Frontend for the uo-link integration, matching the existing site styling.

- api/client.js: api.shard.* (status/feed/economy/idoc/char), the
  shardStreamUrl SSE endpoint, and api.player.shard.* (link/accounts/roster/
  vendors).
- lib/useShardFeed.js: EventSource hook over /public/shard/stream with a
  rolling buffer and a connected flag (browser never touches the sidecar WS).
- routes/public/Shard.jsx: connection banner, stat tiles (online / gold supply
  / link), a gold-supply sparkline, "recent vendor sales" and "IDOC houses"
  lists, and a live event ticker — built from the shared panel/grid/format
  vocabulary. Registered at /site/shard under the maintenance gate and linked
  from the site header.
- routes/player/PlayerAccount.jsx: a "Game accounts" section — enter a [link
  code to link an account, then expand it to see characters and player vendors
  on demand (503 shows a retry banner).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011qPmpmVH1xGCiZoz9m9vW3

client/src/components/SiteHeader.jsx

@@ -3,6 +3,7 @@ import MoonDot from './MoonDot.jsx'
 
 const NAV = {
   website: [
+    { label: 'Shard', to: '/site/shard' },
     { label: 'News', to: '/site/news' },
     { label: 'Screenshots', to: '/site/screenshots' },
     { label: 'Five on Friday', to: '/site/five-on-friday' },