Show per-submission application validation badges

[msaroufim]

Summary

• join the latest validation result for the exact ranked submission and GPU
• only show results matching the leaderboard task's current contract version
• keep validation strictly additive: missing, partial, or failed validation
  never removes or reorders a leaderboard entry
• render VALIDATED, X/Y VALIDATED, or VALIDATION ERROR beside the
  submitter name
• show shape coverage, synchronized-wall speedup, contract version, and check
  time in the tooltip
• fix the leaderboard's error-state precedence and initially-expanded row
  toggle behavior exposed by Leaderboard.test.tsx

Validation

• Leaderboard.test.tsx — 14/14 passed
• production frontend build — passed
• frontend lint and Python Ruff — passed
• API regression coverage proves ranked submission IDs are unchanged for both
  missing and failed validation
• current-tip local Kernelbot + Kernelboard + Postgres + Redis stack rendered
  the attached Seraphim result as 5/8 VALIDATED at 2.82x geomean speedup
• repository-wide Vitest run — 93 passed; 8 unrelated existing assertions
  remain stale in NavBar, News, Login, and NavUserProfile
• production Kernelbot migration and problem sync completed before merge; the
  manual top-10-users B200 sweep completed 10/10 jobs, with coverage ranging
  from 1/8 to 8/8 and one fully validated result

Depends on merged and deployed gpu-mode/kernelbot#518. The Cholesky producer is
merged gpu-mode/reference-kernels#168.