fix(admin): support heading levels 4 through 6

[masonjames]

What does this PR do?

Enables heading levels 4 through 6 throughout the admin content editor. Stored H4–H6 blocks now load correctly, and authors can create them from the heading dropdown, slash commands, or the block transform menu with matching editor spacing.

Closes #2172

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, 1,238 tests passed)
• pnpm format
• New labels and descriptions use Lingui, logical alignment remains RTL-safe, and no catalog files are included.

[changeset-bot]

🦋 Changeset detected

Latest commit: 73ff76b

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

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

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commit: 73ff76b

[emdashbot]

This is the right fix for the reported issue, scoped to the admin editor UI. The backend serialization (packages/core/src/content/converters/*), frontend rendering (PortableText.astro), and the HeadingDropdownMenu component already understand heading levels 1–6. The only gaps were the TipTap StarterKit configuration limiting heading levels to [1, 2, 3], the toolbar passing levels={[1, 2, 3]}, and the slash / block-transform menus not exposing H4–H6. The PR removes those restrictions and adds the missing commands.

I checked the changed code and traces against the existing converters and schema: H4–H6 Portable Text blocks already load and save correctly, so enabling them in the editor is additive and backward-compatible. The new user-facing strings are wrapped with Lingui msg/t, the layout changes use block margins (not directional Tailwind, so RTL-safe), and there are no new queries or server-side changes.

The added tests cover loading an h6 block and exposing H4–H6 in the toolbar, slash menu, and block-transform menu. I did not run the test suite or build (per instructions), but the diff is internally consistent and follows project conventions. No blocking issues.