QuizPanel strands the user on a blank screen when a quiz generates zero questions

[Jose-Gael-Cruz-Lopez]

Severity: Medium
Labels: bug, frontend, P2
Location: frontend/src/components/QuizPanel.tsx:86-95 (start), active-phase guard :202

Description

start() does setQuestions(res.questions || []) then setPhase("active") unconditionally. The active UI is gated {phase === "active" && currentQuestion && ...}; with an empty array currentQuestion is undefined, so nothing renders — no question, no controls (Cancel exists only in the select phase).

Steps to reproduce

1. Start a quiz on a concept the backend can't generate questions for.
2. Land on a blank, control-less panel with no way back except hard navigation.

Expected vs actual

• Expected: an empty-result error and return to select.
• Actual: silent transition to an empty screen.

Suggested fix

In start(), if questions.length === 0, toast.error(...) and stay in select.

Acceptance criteria

• A zero-question generation shows an error and keeps the user in the selectable state.