[lb1192176991-lab]

What does this PR do?

Adds real error states to the dashboard and the quiz builder, so a failed request is no longer shown as "you have no data".

What

• Dashboard.tsx: fetchQuizzes now records a load error; the hosted-quizzes panel shows an error message with a Retry button instead of the empty state, and the play-history panel stays blank instead of claiming "No played history yet." handleCreate records a create error, shown inside the create modal.
• QuizBuilder.tsx: fetchQuiz records a load error, rendered with a Retry button instead of an empty question list; handleGenerateCode records an error shown next to the Generate button.
• Tests: 2 new Dashboard tests and 2 new QuizBuilder tests cover failed load (error + retry + recovery) and failed create / generate-code.

Why

Fixes #180. GET /quizzes and GET /quizzes/:id swallow failures, so a network blip or 500 renders the misleading empty-state copy. An organiser about to host a session is told their quizzes do not exist. The same pattern silently eats failed create and generate-code requests.

Kept each screen's states inline (no shared component) so the diff stays reviewable, per the issue's guidance.

Testing

• npx vitest run src/screens/Dashboard.test.tsx src/screens/QuizBuilder.test.tsx — 11/11 pass.
• Full frontend suite npx vitest run — 34 passed, 1 failed (JoinQuiz.test.tsx); verified that failure exists on clean main without these changes.
• npx tsc -b — clean.
• npx eslint on the touched files — no new warnings; remaining no-explicit-any errors are on untouched lines and pre-exist on main.

Note: this branch is based on main, independent of PR #181 (the duplicate-modal fix). If #181 merges first, I'll rebase.

Related issue

Closes #180

Type of change

• Bug fix
• Tests

Which services does this touch?

• apps/frontend

How did you test this?

Ran the commands above locally on Windows (Node 20, pnpm 10). Simulated failures with mocked API rejections.

Checklist

• I ran pnpm run lint (ran scoped eslint on the changed files instead — see Testing)
• I ran pnpm run check-types (equivalent tsc -b in apps/frontend)
• I ran pnpm run build
• The app still starts with docker-compose up
• I updated documentation if this changes behaviour or setup
• I did not commit any .env file, API key or other secret

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First pull request here? Unticked checkboxes reflect what I could not run on this machine (Docker/full build); happy to run anything else you'd like.