Fix cartoon cut export/upload state and failure handling

[realproject7]

Parent: #197

Found during review of merged PRs #212-#223 before #211 pilot.

Problem

The cartoon cut export/upload flow can leave the UI in an incorrect state or hide failures.

Observed in app/web/components/CutListPanel.tsx and LetteringEditor.tsx:

• LetteringEditor export saves finalImagePath / exportedAt on the backend, but the parent CutListPanel does not reload cuts on successful export or close. The cut can still appear as "Clean ready" and Upload & Generate can remain disabled until a manual reload or unrelated save.
• Export can be run with unsaved overlay edits. The final image contains local overlay state, but cuts.json may not persist those overlays unless the user separately clicks Save.
• Upload & Generate silently continues on failed asset fetch, failed upload, or failed set-uploaded, then generates markdown anyway. The response warnings are not surfaced in that path.

This makes #211 pilot unreliable because the user may think cuts are exported/uploaded when the canonical cut state or generated markdown says otherwise.

Scope

• After successful export, refresh the selected cut / cuts file state in the UI.
• Ensure export either persists current overlays first or clearly requires/surfaces unsaved overlay state before export. Prefer a single user action that leaves cuts.json consistent with the exported image.
• Make Upload & Generate fail visibly if any cut upload step fails. Do not silently skip failed cuts.
• Read and display generate-markdown warnings after Upload & Generate.
• Keep existing PlotLink upload endpoint usage unchanged. Do not add new PlotLink APIs.

Acceptance Criteria

• Exporting a cut updates UI status to lettered without requiring app refresh.
• Exported final image and saved overlay state cannot diverge silently.
• Upload failures show a user-visible error with the failed cut id.
• Markdown generation warnings are shown after upload/generate.
• Upload & Generate only reports success when all required final images are uploaded and recorded in cuts.json.
• Regression tests cover export state refresh, unsaved overlay/export behavior, and failed upload visibility.

Public Safety

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