TASK-012 TUI: Per-source error badge in panel title

Description

When a source has snapshot.ok == false or is missing entirely, panels currently just show "No data." with no indication of why. Distinguish three states visually:

1. Not configured — source disabled or configured: false → dim "—" or italic "not configured"
2. Fetch error — source enabled, snapshot exists, ok: false → ⚠ in border title + error message body
3. No snapshot yet — source enabled but no snapshot row → "waiting for first refresh…"

Implementation:

• In payload(), return a richer sentinel or pass state flags through
• Update each DashPanel's border title with ⚠ prefix when ok: false
• Show the snapshot.error field in the panel body when available

Acceptance Criteria

• #1 Errored sources show ⚠ in panel border
• #2 Error message from snapshot is displayed in panel body
• #3 Not-configured sources are visually distinct from errored sources
• #4 Healthy panels are unchanged