Fix oxlint warnings, extract dialog polyfill, deny warnings in gate

Adds void to floating promise in bestiary-cache.ts, extracts shared
polyfillDialog() helper to eliminate unbound-method warnings in 3 test
files. Adds --deny warnings to oxlint so future warnings fail the
build.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lefthook.yml

@@ -24,6 +24,6 @@ pre-commit:
           - name: typecheck
             run: pnpm exec tsc --build
           - name: oxlint
-            run: pnpm oxlint
+            run: pnpm oxlint -- --deny warnings
           - name: test
             run: pnpm test