[P1] Show actionable data-load errors instead of redirecting away

[kgarg2468]

Problem

Data page sets loadError but immediately router.replace("/data"), which hides actionable failure details and drops project context.

Scope

• Keep user on current project URL when load fails.
• Show persistent error state with retry.
• Add useful error context for debugging.

Acceptance Criteria

• Failed data fetch shows clear message + retry control in-place.
• URL remains project-scoped so retry works without reselecting.

Suggested Files

• app/(app)/data/page.tsx