feat(prose): add table styling support

[amix]

Follow-up to https://github.com/Doist/comms-web/pull/395#discussion_r3395143218 — Comms now renders GFM tables in messages and should eventually use <Prose> instead of its forked prose CSS, but <Prose> had no table styling.

Short description

• Adds bordered table styles to <Prose>, with a new public --reactist-prose-table-border custom property (defaults to var(--reactist-divider-primary)).
• Wide tables scroll horizontally (display: block; width: max-content; max-width: 100%; overflow: auto) instead of stretching the surrounding content, with word-break: normal so cell text doesn't collapse to one character per line under .prose's break-word.
• Headers are bold and start-aligned, with a th:not([align]) guard so explicit column alignment survives both renderer conventions: marked emits the presentational align attribute (which author CSS would override without the guard), while react-markdown emits inline text-align styles (which win regardless).
• Cells reset first/last-child block margins so raw HTML tables with block content (e.g. <td><p>…</p></td>) don't get awkward vertical spacing.
• Adds --reactist-prose-table-header-fill, --reactist-prose-table-row-odd-fill, and --reactist-prose-table-row-even-fill custom properties (all default to transparent) so consumers can color the header row and stripe body rows, demoed in a "Table colors" story.
• Adds a table (with column alignment) to the Storybook prose example, plus a dedicated width-constrained "Table overflow" story so Chromatic exercises the horizontal-scroll layout at any viewport.

Smoke tested in Storybook in light and dark playground modes and in the overflow story.

Out of scope

• Keyboard-focusable scrolling for clipped tables in Safari: Prose is CSS-only over HTML it doesn't control, so it can't inject a focusable wrapper or tabindex. Chrome 130+/Firefox handle this natively for scrollers without focusable children; the wrapper belongs at the consumer's renderer level (tracked for the Comms migration). See the resolved review thread for details.

PR Checklist

• Added tests for bugs / new features (visual coverage via the new Chromatic stories; no unit-testable logic)
• Updated docs (storybooks, readme)
• Reviewed and approved Chromatic visual regression tests in CI

🤖 Generated with Claude Code

[doistbot]

This PR introduces robust table styling support to the Prose component, including borders, explicit alignments, and horizontal scrolling for wide tables.

Few things worth tightening:

• Wrapping the table in a focusable overflow container to ensure horizontal scrolling is accessible to keyboard users.
• Adding a wider table to the Storybook fixture so Chromatic exercises the new overflow layout path.

_{Share Feedback • Review Logs}

[rfgamaral]

Looks good to me 🚀

But here's one suggestion: add more --reactist-prose-* variables that allow consumers to configure the background color of the header row, and the background color of odd and even rows. We can probably leave the Reactist defaults as transparent/inherit/initial (ask your clanker which one is most suited).

[amix]

@rfgamaral great suggestion, I've implemented it. Could you please re-check 🙏

[rfgamaral]

Looks great, merged it for you, @amix. We should get an automated release of this very soon.

[doist-release-bot]

🎉 This PR is included in version 33.1.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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