Add a compact text style selector to Content - #2601

Problem

The Content selection toolbar exposed four permanent H1-H4 buttons but no way to return selected text to regular body text. The four controls also consumed a disproportionate amount of the compact toolbar.

Approach

• Replace the four heading buttons with one labeled selector that reflects the current block style.
• Offer Text and exact H1-H4 choices in a compact accessible menu, with the active choice checked.
• Preserve the selected editor range while the menu has focus so pointer and keyboard choices apply immediately and current-style choices remain idempotent.
• Preserve compatibility with existing H5/H6 blocks: the trigger identifies their actual style and Text converts them back to body text, while H5/H6 remain intentionally absent from the new menu.
• Keep the rest of the formatting toolbar unchanged. Highlight colors are intentionally outside this PR.

Matched visual evidence

Before

Route: http://127.0.0.1:3317/page/j4a5KAEDBHXY
Viewport: 1496 x 846, desktop Chrome
Fixture: disposable local account toolbar-demo@example.com; page Toolbar comparison; paragraph Select this sentence to compare the formatting toolbar.
State: Personal expanded, agent panel closed, full paragraph selected, formatting bubble open, no submenu.

After

Route: http://127.0.0.1:3317/page/vDisteHisXMq
Viewport: 1496 x 846, desktop Chrome
Fixture: same account, page title, paragraph, sidebar state, and agent-panel state. The route differs because the isolated worktree uses its own disposable local fixture database.
State: full paragraph selected, formatting bubble open, Text selector expanded, Text checked, H1-H4 visible.

The matched captures and their reproduction notes are retained in templates/content/docs/solutions/evidence/content-text-selector/.

Verification

• pnpm --filter content exec vitest --run app/components/editor/BubbleToolbar.test.tsx — 8 tests passed, including existing H5/H6 identification and conversion plus a live paragraph-to-H5 transition
• pnpm --filter content typecheck — passed
• pnpm --filter content build — passed; existing non-blocking doctor findings remain
• pnpm guards — all checks passed
• Alice manually exercised the selector in the local Content UI with the final code state and accepted the tested behavior
• Earlier independent browser QA covered pointer conversion, keyboard open/dismiss/selection, idempotent Text, accessibility semantics, clean console, and fixture restoration; the compatibility repair is additionally covered by the exact-head focused tests above

Safety and review focus

• No data model, API, migration, permission, feature-flag, or deployment changes.
• Rollback is limited to the toolbar component, its focused tests, the changelog entry, and visual evidence.
• Please focus review on menu focus/selection preservation, pointer and keyboard behavior, H5/H6 compatibility, and the compact toolbar layout.

Visual-review handoff

Steve's current guidance is to share screenshots of prominent UX changes asynchronously in #dev-agent-native; it explicitly does not need to block shipping. Guidance thread: https://builder-internal.slack.com/archives/C0AKU9QD539/p1784744132830489?thread_ts=1784744132.830489&cid=C0AKU9QD539

Original selector/highlight feedback: https://builder-internal.slack.com/archives/C0ATH3CCZT4/p1785450194911999

This PR prepares the evidence for that later review. It does not claim Slack visual approval, and it does not include the separate Notion-style highlight-color work or the later H5/H6-disable idea.

content_product_impact:
  lane: local_refinement
  features:
    - content.feature.durable-foundations
  capabilities:
    - content.author.document-editor
  record_change: none
  proof:
    - pnpm --filter content exec vitest --run app/components/editor/BubbleToolbar.test.tsx
  rationale: The change refines the existing editor toolbar without changing the document-editor contract.