[UX] Chain Plot — disable textarea and button when no storyline selected

[realproject7]

Problem

When no storyline is selected (or none exist), the "Next Chapter" textarea and "Chain Plot" button are still enabled. Users can type content that can't be submitted.

Expected behavior

• When storyline dropdown shows "Select a storyline" or "No active storylines":
  • Textarea should be disabled (greyed out, not editable)
  • "Chain Plot" button should be disabled
  • Optionally show helper text: "Select a storyline above to chain a plot"

Acceptance Criteria

• Textarea disabled when no storyline is selected
• "Chain Plot" button disabled when no storyline is selected
• Visual feedback (reduced opacity) on disabled elements
• Works for both "no storylines exist" and "none selected yet" states

Files

• src/app/chain/page.tsx