notion-react: add coverage for nested children on list-ish blocks

Part of v0.1 block-coverage epic (see context/notion-block-coverage/).

Problem

The host reconciler (packages/@overeng/notion-react/src/renderer/host-config.ts) hard-codes children -> rich_text for every block in TEXT_LEAF (paragraph, headings, quote, callout, code, bulleted_list_item, numbered_list_item, to_do, table_row, plus toggle via title). Authors cannot nest blocks under list items, to_dos, callouts, quotes, or toggles — making idiomatic Notion trees unreachable from JSX.

Deliverables

• Host (components/blocks.tsx + renderer/host-config.ts): split rich_text children from block children so list-ish containers accept both. Add nested-children reconciliation for: bulleted_list_item, numbered_list_item, to_do, toggle, quote, callout.
• Web mirror (web/blocks.tsx): render nested children in the DOM.
• CSS (web/styles.css): ensure nested indent is styled.
• E2E (test/integration/blocks.integration.test.tsx): at least one nested case per container.

Acceptance

• Nested list trees round-trip Notion -> JSX -> Notion without data loss.
• Existing rich-text-only tests still pass.

Start with packages/@overeng/notion-react/src/renderer/host-config.ts and components/blocks.tsx to trace how rich_text and block children are reconciled. Compare web/blocks.tsx and web/styles.css, then use test/integration/blocks.integration.test.tsx for cases covering each listed container. Done means nested children round-trip without data loss and existing rich-text-only tests pass.