feat: add markdown text card type for dashboards

[aalvaropc]

Summary

• New markdown card type that renders text content without a query
• Content stored in config["content"] — no saved_query needed
• Basic heading support: # H1, ## H2, ### H3 rendered with appropriate font sizes
• Content textarea in the add-card modal when selecting "Text / Markdown" type
• Markdown cards skip query execution (no async tasks, no loading state)
• Available in show, public, and embed dashboard views

Test plan

• mix precommit — 638 tests, 0 failures
• Manual: add a markdown card with headings and text, verify it renders without a query