Render HTML tables in the pull request reader #1007

Since #1000 the pull request reader converts the HTML review bots write — inline formatting, <a>, <img>, <br>, <pre>, <details> — but a <table> still renders as its source text. Review bots lean on tables heavily ("PR Reviewer Guide" grids, per-file summaries), so it is the most visible gap left.

The pass (src/Capacitor.App/GitHubHtml/) rewrites HTML into Markdig nodes and lets MarkView render them; the natural target is Markdig's Table/TableRow/TableCell, which MarkView already draws as a grid (pipe tables use it today). What the block rule needs: table, thead, tbody, tr, th, td as structural tags with their own nesting discipline, cell content built through the existing InlineBuilder, colspan/rowspan either refused (whole block stays source) or flattened, and the all-or-nothing rule kept so a half-understood table never shows.

Design notes that still apply: docs/superpowers/specs/2026-09-17-ai2889-github-markdown-pr-reader-design.md (D3 block rule, depth budget, degrade rule).