BlockRenderer, postContent: parse plain-text markdown tables in messages

[jamesacklin]

Summary

Render GFM-style Markdown tables in messages as real tables instead of pipe-text. Detection runs client-side at receive time — wire format unchanged, old clients still see plaintext.

Why is this PR so big?

About 1,300 lines net but most of it is moved code, not new logic. Here's how to read it:

~870 lines of genuine new code — start here:

• extractTables.ts (~415 lines) — the splitter
• extractTables.test.ts (~290 lines) — 9 unit tests for the splitter
• groupMentionPlugin.ts (~85 lines) — mirrors shipMentionPlugin.ts
• postContentInlines.ts (~80 lines) — convertInlineContent extracted out of postContent.ts to break a cycle between convertContent and extractTablesFromContent

~160 lines of additions to existing files:

• BlockRenderer.tsx — the TableBlock component (column-grouped layout, three-phase onLayout measurement)
• postContent.ts — TableBlockData type, BlockData union extension, plaintextPreviewOf case, single call to extractTablesFromContent at the end of convertContent
• mdastToStory.ts — isGroupMention type guard and handler in phrasingToInlines

The rest is a directory move (packages/shared/src/logic/markdown/ → packages/api/src/client/markdown/).

Changes

• New TableBlockData block type + TableBlock renderer (column-grouped layout, two-pass onLayout measurement, capped at 280px wide, header-row dim styling).
• Splitter at packages/api/src/client/markdown/extractTables.ts finds GFM tables in paragraph content, parses with remark-gfm, emits TableBlockData. Uses mdast-util-to-markdown for serialization so bold/links/mentions in cells survive intact.
• New remarkGroupMentions plugin (parallel to remarkShipMentions) so @all / @admin round-trip through table cells.
• Moved the entire markdown module from packages/shared/src/logic/markdown/ → packages/api/src/client/markdown/. api now owns content conversion end-to-end; convertContent calls extractTablesFromContent directly.

How did I test?

• 9 new unit tests in extractTables.test.ts covering simple extraction, soft-wrap continuation merging, no-outer-pipe tables, bold/link/group-mention preservation in cells, and pre/post-paragraph splitting.
• All 50 existing markdown tests + 51 storyToMarkdown tests + 58 roundTrip tests still pass.
• Typecheck clean for files in this branch.
• pnpm lint passes with zero errors (117 warnings, all pre-existing on develop).
• Verified visually with a bot post whose cells contained bold + link inlines.

Risks and impact

• Safe to rollback without consulting PR author? Yes
• Affects important code area:
  • Onboarding
  • State / providers
  • Message sync
  • Channel display
  • Notifications
  • Other:

No wire-format or backend changes. New code path only fires on paragraphs whose text matches a GFM table signature.

Rollback plan

Revert the branch. To narrowly disable detection without removing the type, remove the extractTablesFromContent call at the end of api's convertContent.

Screenshots / videos

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[claude]

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[jamesacklin]

@claude review once

[patosullivan]

Overall looks good, but there are a few issues I think should probably be addressed before we merge. Most are pretty minor.

[patosullivan]

lgtm