feat(schema): multiple option for image/file fields (ordered media galleries)

[Rimander]

What does this PR do?

Adds a multiple option (stored as validation.multiple) to image and file fields so editors can manage an ordered set of media on one entry — photo galleries, slideshows, download lists — instead of a single item. This is the "multiple: true modifier on existing image/file fields" shape requested for gallery-style content, chosen over a new gallery field type: the column type is unchanged (TEXT storing JSON), so existing fields can be upgraded in place with no migration, and the flag composes with allowedMimeTypes, minItems/maxItems.

Core:

• FieldValidation.multiple + API zod acceptance (fieldValidation), so the flag survives create/update field calls.
• Zod generator wraps the image/file item schema in z.array() when multiple is set, applying minItems/maxItems. Toggle round-trips are safe both ways: a stored single object validates (wrapped into a one-item array) after enabling multiple, and a one-element array unwraps after disabling it (Auto save validation errors emdash-cms/emdash#867-class autosave protection).
• Save-side MIME allowlist validation checks every item of the array (previously arrays were silently skipped).
• Media-usage extraction records one occurrence per item with indexed field paths (gallery[0], gallery[1], …).
• normalizeMediaFields enriches each array item from the media provider.
• Generated TypeScript types emit Array<{...}> for multiple fields.

Admin:

• Field editor gains an "Allow multiple images/files" switch with Min/Max Items, plus a warning when disabling it on an existing field.
• MediaPickerModal gains an additive multi-select mode (multiple + onSelectMany); single-select behavior is untouched.
• New MultiMediaFieldRenderer renders the ordered list with drag-and-drop reordering (same dnd-kit pattern as RepeaterField) for both image and file fields.

Also fixes a corrupt Arabic translation ({0, plural, one {(# item)} other {(# items)}} had unbalanced ICU braces) that crashed any component rendering that count badge in Arabic — including the pre-existing RepeaterField. This is a surgical one-line repair of a broken catalog entry, not extraction churn; without it the new gallery renderer (which reuses the same msgid) crashes for Arabic users. Reproduced and verified with @lingui/core directly.

Closes #

Type of change

• Bug fix
• Feature (requires maintainer-approved Discussion)
• Refactor (no behavior change)
• Translation
• Documentation
• Performance improvement
• Tests
• Chore (dependencies, CI, tooling)

Checklist

• I have read CONTRIBUTING.md
• pnpm typecheck passes
• pnpm lint passes
• pnpm test passes (or targeted tests for my change) — full core suite (4690 tests) and admin suite (1100 tests)
• pnpm format has been run
• I have added/updated tests for my changes (if applicable)
• User-visible strings in the admin UI are wrapped for translation (if applicable). The ar/messages.po diff is a deliberate one-line fix of a corrupt ICU plural entry (see description), not extracted catalog churn.
• I have added a changeset
• New features link to an approved Discussion: to be filled in before submitting upstream — this PR is on the fork; the feature responds to a downstream gallery/multi-media request (no upstream Discussion link yet)

AI-generated code disclosure

• This PR includes AI-generated code — model/tool: Claude Code + Claude Fable 5

Screenshots / test output

Verified end-to-end in the demos/simple dev app (Node + SQLite):

• Created an image field with "Allow multiple" in the field editor; validation persisted as {"multiple": true} (type stays image, column stays TEXT).
• Selected 3 images at once in the media picker, saw ordered thumbnail rows, drag-reordered, removed one; each state autosaved and the API returned the gallery as an ordered array reflecting every change.
• Generated demo types picked up gallery?: Array<{ id: string; ... }>.
• Tested the editor in Arabic (RTL): rows mirror correctly (thumbnail right, remove button left) and the item-count badge renders Arabic plurals after the catalog fix.

🤖 Generated with Claude Code

[github-actions]

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Warnings reference

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🔄️	The source for this localization has been updated since the creation of this pull request, make sure all changes in the source have been applied.