feat(media) Add media drag uploads and picker metadata editing

[masonjames]

What does this PR do?

Adds the next media workflow improvements from discussion #990:

• Adds drag-and-drop batch uploads to the Media Library and Media Picker.
• Reuses the existing upload paths with shared batch progress and error handling.
• Adds active/reject drop states, upload progress, success/error messages, and invalid-file rejection.
• Keeps picker uploads selected after successful upload, including provider uploads.
• Adds inline alt text and caption editing in the Media Picker using the existing updateMedia behavior.
• Adds missing-alt, alt-set, and caption-set indicators plus quick edit affordances.

Related discussion: #990

Closes: n/a (discussion-driven feature)

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)
• 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). Do not include messages.po changes except in translation PRs — a workflow extracts catalogs on merge to main.
• I have added a changeset (if this PR changes a published package)
• New features link to an approved Discussion: Make media handling a first-class editing workflow #990

AI-generated code disclosure

• This PR includes AI-generated code — model/tool: GPT-5 Codex, RepoPrompt workflow tools

Screenshots / test output

Manual QA completed locally before PR creation; optional RTL check was not rerun manually. The implementation was also smoke-checked in Arabic/RTL during development.

Automated checks run locally:

• pnpm format
• pnpm lint:json | jq '.diagnostics | length' -> 0
• pnpm typecheck -> passed
• pnpm --filter @emdash-cms/admin test tests/components/MediaLibrary.test.tsx tests/components/MediaPickerModal.test.tsx --run -> 64 passed
• pnpm --filter @emdash-cms/admin build -> passed

[changeset-bot]

🦋 Changeset detected

Latest commit: b02fa2f

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 14 packages

Name	Type
@emdash-cms/admin	Minor
emdash	Minor
@emdash-cms/cloudflare	Minor
@emdash-cms/sandbox-workerd	Patch
@emdash-cms/fixture-perf-site	Patch
@emdash-cms/perf-demo-site	Patch
@emdash-cms/cache-demo-site	Patch
@emdash-cms/auth	Minor
@emdash-cms/blocks	Minor
@emdash-cms/gutenberg-to-portable-text	Minor
@emdash-cms/x402	Minor
create-emdash	Minor
@emdash-cms/auth-atproto	Patch
@emdash-cms/plugin-embeds	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[github-actions]

Scope check

This PR changes 2,259 lines across 8 files. Large PRs are harder to review and more likely to be closed without review.

If this scope is intentional, no action needed. A maintainer will review it. If not, please consider splitting this into smaller PRs.

See CONTRIBUTING.md for contribution guidelines.

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commit: b02fa2f