Add cartoon clean image asset management and cut list UI

[realproject7]

Parent: #197

Proposal reference: ~/z-docs/PROPOSAL-plotlink-ows-webtoon-pivot.md

Context

The agent produces or helps import clean images. OWS needs a way to associate those clean images with cuts before lettering.

Scope

• Add a cartoon cut list UI for selected cartoon plots.
• Support assigning/importing a clean image to a cut.
• Store clean image paths in plot-NN.cuts.json.
• Keep assets under a plot-local folder such as plot-01.assets/.
• Show cut status: missing clean image, clean ready, lettered/exported, uploaded.

Acceptance Criteria

• User can select a cartoon cut and attach a clean WebP/JPEG image.
• Asset paths are stored relative to the story folder.
• Existing plot illustration upload UI is not broken.
• Tests cover clean image assignment and missing-image states.

Public Safety

This is a public repo. Do not include secrets, API keys, wallet private keys, mnemonics, auth tokens, OWS vault internals, private story drafts, or sensitive local paths in issues, PRs, logs, docs, tests, or examples.