feat(dashboards): add section headers

[lordspline]

Problem

Dashboards currently support text tiles, but not a first-class way to add lightweight section breaks. Users organizing dense dashboards need a section header that behaves like a dashboard tile without introducing a new dashboard model or migration.

Changes

• Add an Add → Section header flow that reuses the generated dashboard text-tile create/update APIs and persists as marked text tile content with transparent background.
• Render section headers as plain text title/description rows with a transparent divider treatment, full-width default placement, mobile-friendly height, edit/duplicate/delete support, and dashboard-template serialization.
• Extend text tile create/update serializers with transparent_background, regenerate dashboard/MCP API types, and report text tile creation through the existing dashboard tile added action path.
• Screenshots/video captured locally in the agent session: section-header-treatment-a.png, section-header-treatment-b.png, section-header-browser-before.png, section-header-browser-after-add.png, section-header-browser-after-drag.png, section-header-dashboard-flow.mp4.

How did you test this code?

I’m an agent. I tested with:

• pnpm --filter=@posthog/frontend format
• pnpm --filter=@posthog/frontend typescript:check
• hogli test frontend/src/lib/components/Cards/TextCard/textCardSectionHeader.test.ts frontend/src/lib/components/Cards/TextCard/TextCard.test.tsx frontend/src/lib/components/Cards/TextCard/textCardModalLogic.test.ts frontend/src/scenes/dashboard/DashboardHeader.test.tsx frontend/src/scenes/dashboard/EmptyDashboardComponent.test.tsx frontend/src/scenes/dashboard/dashboardLogic.test.ts frontend/src/scenes/dashboard/dashboardUtils.test.ts frontend/src/scenes/dashboard/tileLayouts.test.ts
• hogli test posthog/api/test/dashboards/test_dashboard.py::TestDashboard::test_create_text_tile_adds_markdown_tile_to_dashboard posthog/api/test/dashboards/test_dashboard.py::TestDashboard::test_create_text_tile_reports_tile_added_event posthog/api/test/dashboards/test_dashboard.py::TestDashboard::test_create_text_tile_requires_edit_permissions posthog/api/test/dashboards/test_dashboard.py::TestDashboard::test_update_text_tile_updates_body_and_layout posthog/api/test/dashboards/test_dashboard.py::TestDashboard::test_update_text_tile_requires_edit_permissions posthog/api/test/dashboards/test_dashboard.py::TestDashboard::test_duplicate_text_tile_within_dashboard_preserves_section_header_fields posthog/api/test/dashboards/test_dashboard.py::TestDashboard::test_create_from_template_json_can_provide_text_tile
• Browser verification on a seeded dashboard: add a section header, name it, drag it to a new row, edit it, and confirm the stored layout changed.

👉 Stay up-to-date with PostHog coding conventions for a smoother review.

Automatic notifications

• Publish to changelog?
• Alert Sales and Marketing teams?

Docs update

No docs update included.

🤖 Agent context

Autonomy: Human-driven (agent-assisted)

Captain Capy implemented this as a fork experiment inspired by issue PostHog#58307. I prototyped a transparent divider treatment and a subtle accent-band treatment against realistic dashboard fixtures, then chose the transparent divider because it fits existing dashboard/text-tile visual language without competing with chart cards.

The implementation intentionally avoids a new dashboard model or migration. It uses existing text tile persistence with a marker in the body, transparent_background, generated dashboard APIs, and layout defaults to preserve section-header behavior across creation, editing, duplication, templates, and responsive dashboards.

---

^{Need help on this PR? Tag /codesmith with what you need. Autofix is disabled.}