fix: surface errors on moderation actions instead of silently closing dialog

[sandeep-2536]

What was broken

doAction in AdminModeration.tsx silently caught and discarded API errors.
When a warn/suspend/ban request failed, the modal still closed as if it had
succeeded — so an admin had no way to know the action didn't actually happen.

Fix

• Errors are now surfaced to the admin instead of being swallowed
• The modal/dialog only closes on confirmed success; it stays open on failure
  so the admin can retry, and the entered reason text is preserved
• Added AdminModeration.test.tsx covering both the failure path (modal stays
  open, error shown) and the success path (modal closes, fields reset)

How this was found

Found via manual code review of the admin moderation flow, not from an
assigned GitHub issue.

Testing

• Manually reproduced the failure path by blocking the moderation request in
  DevTools — confirmed error is shown and modal stays open
• Confirmed the success path still works normally after unblocking
• pnpm exec tsc --noEmit passes
• pnpm test — new AdminModeration.test.tsx tests pass. Note: 3 unrelated
  test suites (api.test.ts, PremiumTee.test.ts, GoldenTicketDetailPage.test.tsx)
  fail on main as well, due to a pre-existing tooling error
  (jsTokens is not a function) unrelated to this change.