NO_TESTS verdict renders as a bogus 'Runtime Error' with a broken badge

**Severity:** Medium A problem seeded without hidden tests returns `verdict: "NO_TESTS"` (`src/server/judge.ts:149`). The submit route passes it through, but the UI has no branch for it, so it falls through to the WA/RE default and `VerdictBadge` shows an empty/broken chip. **Where** - `src/server/judge.ts:149` - `src/components/cp-arena/ArenaWorkspace.tsx` (verdict handling + `VerdictBadge` map) **Fix** Handle `NO_TESTS` explicitly in the UI (e.g. an "infrastructure / no tests yet" message) and add it to the `VerdictBadge` map. **Acceptance:** submitting to a problem with no hidden tests shows a clear message, not "Runtime Error".