fix(book): every image frame shows a different photo

[patriksimms]

Problem

As an organizer I want each image frame in a layout to show a different contributor photo, so a page with three frames tells a story instead of printing the same photo three times.

Every frame bound to a question rendered images[0], in three separately derived places. On top of that, preflight compared the photo count against a single element's slots, so an image frame on a question that allows two uploads raised a blocking gallery-overflow problem and stopped the export outright.

Solution

One shared assignment function (src/domain/photo-assignment.ts) maps a response's photos to the frames bound to each question, in visual reading order (top to bottom, then left to right, element id as final tiebreak). Preview, PDF export, and preflight all read from it, so a preflight warning always describes the photo that actually prints, and the mapping is stable across regenerations and relayering.

A gallery consumes its slots as one contiguous block at its position in that order.

Photo/slot mismatches in either direction now raise one non-blocking, question-scoped photo-slot-mismatch page problem instead of blocking export, and the layout editor flags a slot count that does not match a question's maximum image count while the layout is being designed.

Closes #60

Notes for review

• The first commit is a CI fix, not part of the feature. verify has failed on main since ddd2194: the RustFS bind mounts create .local/ as root, after which the workflow user cannot create .local/icc. Running setup:icc before docker compose up restores it. Without it this branch could not reach a green gate.
• gallery-overflow stays accepted by the persisted-book validator so books written before the rename remain loadable and re-saveable; nothing produces it any more.
• A frame that stays empty now raises a warning even when the contributor uploaded no photos for an optional question. That follows the issue directly and surfaces blank frames during review, but it does make optional photo questions noisier on the review screen.
• The layout parity baselines were regenerated: the fixture binds an image frame and a gallery frame to the same question, so it now demonstrates the distribution.

Validation

bun run verify locally (format, lint, typecheck, 231 unit tests, 21 Playwright tests, build, compose checks) and green CI on 37f7fd7.

Before and after screenshots are attached to #60.

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Delivered by Claude Opus 5 (1M context) in Claude Code.