Feature: Improve teacher feedback for fetch failures and empty classrooms

[NewtonLC]

Description

PR #603 fixed a crash that occurred when the student data server was offline. The fix returns [] on any fetch failure, which prevents the crash — but leaves the teacher with a blank dashboard and no explanation. This issue tracks two follow-on UX improvements that build on that fix.

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Problem 1 — The teacher can't tell if the fetch failed

fetchStudentData() currently returns [] for three distinct situations:

Case	Cause	Relevant long-term?
Missing env var	MOCK_USER_DATA_URL not set	No — dev/config only
Bad HTTP response	Server returned 4xx/5xx	Yes
Network error	Server unreachable	Yes

The calling code in pages/dashboard/[id].js receives [] in all cases and passes it straight to DashTabs as data. There's no way to distinguish a failed fetch from an empty class.

Proposed fix:

Return a structured result from fetchStudentData() instead of a bare array:

// Success
return { error: null, data: await response.json() };

// Runtime failures (surface to teacher)
return { error: 'FETCH_FAILED', status: response.status, data: null };
return { error: 'NETWORK_ERROR', data: null };

// Config issue (dev only — keep silent, log only)
console.warn('MOCK_USER_DATA_URL is not defined.');
return { error: 'MISSING_URL', data: null };

[id].js would pass the error code as a prop. For runtime failures, the dashboard UI would render an error message: "We couldn't load your students. Please try refreshing, or contact support at support@freecodecamp.org if the problem persists." The MISSING_URL case should remain a silent console log, as it's dev-facing.

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Problem 2 — Empty classrooms show nothing

When a class has no students yet, DashTabs renders a blank page:

Proposed fix:

In getServerSideProps in [id].js, include fccUserIds in the classroom query. If it's empty, the dashboard should not render an empty table. Instead, it should:

• Tell the teacher there are no students in the class yet
• Display the classroom's join link with a copy button
• Instruct the teacher to share the link with their students

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Files likely affected

• util/student/fetchStudentData.js
• pages/dashboard/[id].js
• The component(s) rendered by DashTabs that display student data
• prisma query in pages/dashboard/[id].js (add fccUserIds to the select)