fix(admin): add subscript and superscript formatting

[masonjames]

What does this PR do?

Adds subscript and superscript marks to the admin content editor. Both marks are available from the fixed toolbar and text-selection bubble menu, remain active while editing, and round-trip through Portable Text without being dropped.

Closes #2144

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: Not applicable; this is a maintainer-approved bug fix.

AI-generated code disclosure

• This PR includes AI-generated code — model/tool: OpenAI Codex (GPT-5)

Screenshots / test output

• pnpm typecheck
• pnpm lint
• pnpm --filter @emdash-cms/admin exec vitest run (104 files, 1,239 tests passed)
• pnpm format
• User-facing labels are wrapped with Lingui; no catalog files are included.

[changeset-bot]

🦋 Changeset detected

Latest commit: acece22

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

This PR includes changesets to release 17 packages

Name	Type
@emdash-cms/admin	Patch
emdash	Patch
@emdash-cms/cloudflare	Patch
@emdash-cms/sandbox-workerd	Patch
@emdash-cms/plugin-mcp-smoke	Patch
@emdash-cms/fixture-perf-site	Patch
@emdash-cms/perf-demo-site	Patch
@emdash-cms/cache-demo-site	Patch
@emdash-cms/do-demo-site	Patch
@emdash-cms/do-solo-demo-site	Patch
@emdash-cms/auth	Patch
@emdash-cms/blocks	Patch
@emdash-cms/gutenberg-to-portable-text	Patch
@emdash-cms/x402	Patch
create-emdash	Patch
@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

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commit: acece22

[emdashbot]

This is the right fix for #2144: it wires official TipTap subscript/superscript extensions into the admin editor the same way bold/italic/strike are already handled, and it plumbs the marks through both directions of the Portable Text converter so they are not dropped on round-trip.

What I checked:

• Dependencies and catalog entries are correctly pinned to the same ^3.20.0 TipTap version as the other editor extensions.
• Subscript and Superscript extensions are registered in the TipTap editor and exposed from both the fixed toolbar and the text-selection bubble menu.
• The activeMarks / editorState selectors reference the correct mark names (subscript/superscript), so button pressed state updates live.
• The PM → PT converter maps the new marks explicitly, and the PT → PM converter emits the matching ProseMirror marks; without the new case branches the marks would fall through the default and be lost.
• Toolbar/bubble buttons use t\Subscript`/t`Superscript`for Lingui and includearia-label/aria-pressed`.
• Tests cover rendering from PT marks to <sub>/<sup> and persisting the marks through onChange after toolbar activation.
• The changeset is scoped to @emdash-cms/admin and describes the user-visible change.

One non-blocking observation, outside the admin scope of this PR: the core Portable Text converters in packages/core/src/content/converters/ still fall through their default mark handler on the PT → PM path, so marks named subscript/superscript would be dropped there. The admin editor uses its own duplicated converters, so this PR is self-contained and correct, but you may want to mirror the two new cases in the core converters later for any template/plugin code that consumes the public API.

Nothing needs to change before merge.