feat: add compact workspace composition primitives

[karngyan]

Summary

• add PageHeader with compact title, context, actions, and secondary navigation slots
• add SettingsSection and SettingsRow with divider-led preference layout
• add dedicated documentation, catalog previews, generated agent docs, and a minor changeset
• verify phone geometry without adding duplicate segmented control or pane APIs

Validation

• pnpm test, 266 passed
• pnpm typecheck
• pnpm format:check
• pnpm --filter @dowel/docs build
• pnpm test:a11y, 15 passed
• Playwright rendering at 390px and 1440px

Stack

Depends on #15, which depends on #14 and #13.