Add Arena blind battles, model-grouped Demo Show, and a community leaderboard

Each case now defaults to Arena mode: two anonymous artifacts (identities
hidden, footers suppressed) battle via A / B / Neither, reveal after the
vote, with the same clip-reveal motion language as the rest of the site.
Demo Show replaces the per-combination tab strip with one tab per model;
multi-harness models get a harness switcher and a side-by-side compare,
so the lineup no longer bloats as harnesses multiply.

Votes live in lib/arena.ts (localStorage, custom event fan-out) and rank
via Bradley-Terry fits on pairwise wins, LMSYS Chatbot Arena style, with
'neither' surfaced as a both-bad rate. The #board section stays locked
until 3 votes are cast in each case (blind first, board after), then
shows your votes plus the ranking. When VITE_ARENA_API is configured the
store syncs to the zero-dependency Cloudflare Worker + D1 backend in
worker/ (Turnstile verification, per-IP daily caps, idempotent retries);
deploy steps live in worker/README.md.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.github/workflows/deploy.yml

@@ -26,6 +26,10 @@ jobs:
       - name: Build
         run: bun run build
         working-directory: site
+        env:
+          # Optional arena backend (see worker/README.md); empty = local-only votes.
+          VITE_ARENA_API: ${{ vars.VITE_ARENA_API }}
+          VITE_TURNSTILE_SITE_KEY: ${{ vars.VITE_TURNSTILE_SITE_KEY }}
       - uses: actions/configure-pages@v5
       - uses: actions/upload-pages-artifact@v3
         with: