fix: Prevent draft autosave state loss and editor overwriting

[orrc]

What does this PR do?

Fixes a cluster of related draft/autosave bugs in the admin editor for revision-enabled content.

This started from a verified repro on a published post:

• Editing "Content" and waiting for autosave appeared to work locally
• However, the autosave PUT response returned stale content
• A redundant follow-up GET request also returned stale base-row data
• Any edits to "Excerpt" were overwritten immediately after autosave
• After page refresh, the rich text editor appeared to lose the saved draft content even though the draft revision had actually been updated

Root cause turned out to be three separate compounding issues:

1. packages/core

   • revision-backed autosave wrote the draft correctly, but the PUT response was built from the stale base content row instead of the active draft revision

2. packages/admin

   • autosave handling in the edit page was not safe for revision-backed content
   • initially this caused a redundant post-autosave GET
   • more importantly, an autosave response could overwrite newer local edits if the user kept typing while the request was in flight

3. packages/admin

   • PortableTextEditor treated its initial value as mount-only state
   • when parent state changed after mount (for example after draft load, reload, or reset), the live TipTap editor did not reconcile to that new value

What this PR changes:

• Returns the correct, draft-aware data from autosave responses for revision-backed content.
• Removes the redundant post-autosave GET call from the client (as the necessary data is in the autosave response).
• Only applies an autosave response back into edit-page queries when the local editor state still matches the snapshot that was sent.
• Reconciles post-mount external value changes into the rich-text editor.

Relates to #272, but extends beyond that admin-only reset loop to also fix the stale revision-backed autosave response, the redundant post-autosave refetch, and the in-flight autosave overwrite edge case.

Type of change

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

Checklist

• I have read CONTRIBUTING.md
• pnpm typecheck passes
• pnpm --silent lint:json | jq '.diagnostics | length' returns 0
• pnpm test passes (or targeted tests for my change)
• pnpm format has been run
• I have added/updated tests for my changes (if applicable)
• I have added a changeset (if this PR changes a published package)
• New features link to an approved Discussion: https://github.com/emdash-cms/emdash/discussions/...

AI-generated code disclosure

• This PR includes AI-generated code
  But with a lot of prior verification, including with Playwright, and a lot of discussion on the solution

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Latest commit: c3020e0

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[orrc]

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[github-actions]

Overlapping PRs

This PR modifies files that are also changed by other open PRs:

• feat(admin): add i18n with Lingui #234 (3 shared files)
• feat(admin): add Taxonomy Types management page #213 (3 shared files)
• Store CSS classes, make editor toolbar extensible #143 (4 shared files)

This may cause merge conflicts or duplicated work. A maintainer will coordinate.