Show actionable errors for invalid cartoon cuts.json

[realproject7]

Context

During the #211 pilot, the agent generated a custom plot-01.cuts.json schema. The current API correctly rejects invalid cut files through validateCutsFile, but the user-facing cartoon workspace needs to make this failure obvious and actionable instead of leaving the user with an empty or confusing preview/editor state.

Parent epic: #197. Blocks #211 pilot approval.

Task

Improve UI/API handling for invalid cartoon plot-NN.cuts.json files.

Files to inspect/update:

• app/routes/stories.ts
• app/lib/cuts.ts
• app/web/components/CutListPanel.tsx
• app/web/components/PreviewPanel.tsx
• relevant tests under app/routes/*.test.ts and app/web/components/*.test.tsx

Requirements

• When a cartoon plot has an invalid cuts file, surface the exact validation error in the UI.
• Do not silently treat invalid cuts.json as “no cuts found”.
• The error should tell the user/agent that plot-NN.cuts.json must follow the OWS v1 schema.
• If there is a natural existing recovery action, expose it safely. Acceptable MVP recovery options:
  • show “Regenerate cuts.json using the v1 schema” guidance text, or
  • provide a guarded reset/recreate action only if it does not delete existing user-created assets or text without confirmation.
• Keep fiction preview behavior unchanged.

Acceptance Criteria

• Invalid plot-NN.cuts.json produces a visible, actionable error in the cartoon workspace.
• Tests cover invalid JSON and wrong-schema JSON.
• Existing valid cartoon cut flows still work.
• Existing fiction flows still pass.
• npm run typecheck and relevant tests pass.

Public Safety

Do not include private story content, secrets, wallet data, auth tokens, local absolute user paths, or Claude session IDs in tests/docs/issues.