notion-react: add coverage for table / table_row

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

Problem

• table_row currently routes through TEXT_LEAF (rich_text) instead of Notion's cells: rich_text[][].
• table component does not expose table_width, has_column_header, has_row_header.
• No E2E for tables.

Net effect: a table authored through the library is not a valid Notion table.

Deliverables

• components/blocks.tsx: Table props { width, hasColumnHeader, hasRowHeader }. TableRow accepts cells prop (ReactNode[]), each cell flattened to rich_text.
• renderer/host-config.ts: project table/table_row payloads to the Notion shape; remove table_row from TEXT_LEAF.
• web/blocks.tsx: mirror new props (render headers, multi-cell rows).
• web/styles.css: verify .notion-simple-table styling for headers.
• E2E: one table round-trip with 2 cols x 2 rows, column header on.

Acceptance

• Table round-trips without Notion API rejection.
• Cells render per-column in web mirror.

Start in components/blocks.tsx and renderer/host-config.ts to trace Table and TableRow props and the current TEXT_LEAF routing. Then inspect web/blocks.tsx and web/styles.css for the mirrored table behavior and header styling. Done means a 2-column, 2-row table with column headers round-trips without Notion API rejection and renders cells in separate columns in the web mirror.