feat: support header.level (1-4) on header block

[programad]

Summary

Adds support for the level field on the header block. The field is part of Slack's documented public schema as of the 2026-03-06 Block Kit refresh but was missing from this library, so headers all render at one size regardless of intended hierarchy.

From Slack's header block reference:

level | Integer | Optional. Set the level of the heading. Values 1-4 correspond to H1-H4 heading levels, respectively.

What changed

• src/types/layout.ts — extend HeaderBlock with level?: 1 | 2 | 3 | 4 and JSDoc pointing at the Slack docs.
• src/components/blocks/header.tsx — pick the semantic heading tag (<h1> / <h2> / <h3> / <h4>) from level and apply a per-level size class. When level is omitted, rendering is unchanged (<h3> + text-header / 18px), preserving back-compat with the pre-rollout single-size behavior.
• KNOWLEDGE_BASE.md — chapter 4.1 mentions the new level prop.

Back-compat

• level is optional. Existing usage (no level) renders identically to before — same <h3> tag, same text-header styling.
• Setting level: 3 is intentionally equivalent to the legacy default so consumers can opt in incrementally.
• Adds an opt-in data-header-level attribute on the heading element for consumers who want to style further.

Size class choices

I picked reasonable defaults:

level	Tag	Size class
1	<h1>	text-[28px] leading-[1.2]
2	<h2>	text-[22px] leading-[1.27]
3	<h3>	text-header (18px) — matches legacy default
4	<h4>	text-base (15px)
undefined	<h3>	text-header — legacy fallback

The library's existing fontSize tokens (text-header: 18px, text-base: 15px) cover H3 and H4. H1 and H2 use inline arbitrary values since the library didn't have heading-1/heading-2 tokens previously. Happy to switch to dedicated tokens (e.g., text-header-1, text-header-2) in tailwind.config.js if preferred — wanted to keep this PR minimal.

Testing

• pnpm lint (tsc) — clean
• pnpm build (tsup + postcss) — clean; built dist/index.d.ts includes the new level?: 1 | 2 | 3 | 4 field
• Manual exercise via the existing test fixtures TBD by reviewer

Motivation

Apps syndicating rich content into Slack (e.g., a CMS pushing posts with multi-level headings) want their preview to match what Slack actually renders. Slack now distinguishes H1-H4 visually after the 2026-03-06 rollout, but this library still renders every header at the same size, so any preview built on it diverges from the real Slack message. This change closes that gap.

References

• Header block reference — documents level 1-4
• 2026-03-06 Block Kit changelog — rollout context
• Markdown block reference — alternative path; per docs "all header levels are rendered at the same size", so the header block is the correct route for size-distinguished headings