feat(dashboards): add section headers

[lordspline]

Problem

Dashboard editors can add charts and text, but they cannot divide a dashboard into clearly labeled sections.

Changes

• Adds a Section header action with a compact title and optional description form.
• Defaults section headers to a full-width, one-row layout on desktop and mobile dashboards.
• Preserves edit, duplicate, move, copy, delete, undo, drag, and resize behavior.
• Stores section metadata in existing text tiles with an escaped Markdown fallback.
• Reuses text-tile APIs and permissions, so this needs no model or migration.

Open video

How did you test this code?

• Focused Jest tests guard encoding, validation, create and edit payloads, responsive defaults, literal rendering, and tile actions.
• The dashboard logic suite guards existing tile behavior, including deletion and undo.
• Django tests guard create, retrieve, edit, and template JSON round trips through the public dashboard API.
• Browser QA covered create, edit, duplicate, delete, undo availability, responsive rendering, and drag reordering.
• Frontend formatting, linting, type checking, and ci:preflight --fix completed successfully.

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Automatic notifications

• Publish to changelog?

Docs update

No matching dashboard workflow documentation exists for this discoverable action.

🤖 Agent context

Autonomy: Human-driven (agent-assisted)

• Capy implemented and verified the change in this thread.
• Skills invoked: /writing-ui-components, /writing-tests, /writing-user-facing-copy, /writing-code-comments, /writing-pr-descriptions, and /running-ci-preflight.
• The implementation reuses text-tile persistence instead of adding a dashboard model or migration.
• All committed fixtures and published media use local demo or invented data.

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[github-actions]

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