feat(selfupdate): render changelog inline in update modal

[frahlg]

Summary

• Fetch the GitHub release body alongside tag + notes URL. Cap at 16 KiB so a runaway changelog can't bloat `/api/version/check`.
• Render markdown inline in the update modal as a collapsible `
  Details

  ` (default-open) so operators see what's changing before pressing Update.
• Tiny allowlist markdown renderer (headings, bullets, bold, inline code, links via `safeHref`). Everything else stays escaped plain text.
• `Open on GitHub ↗` stays as a fallback for the full formatted view.

Why

Operators asked: "I want to read the changelog before I click Update, not after." The old modal only had a tiny link to GitHub — clicking it opens a new tab, which breaks the flow when all you want is a 5-second glance at what's in the release.

Safety

• Release body is escaped first, then markdown tokens are rewritten. Untrusted content can't inject HTML.
• Link URLs route through the existing `safeHref` gate, which rejects anything that isn't `http:` / `https:`.
• Body is capped at 16 KiB with a `…(truncated — see release notes for full changelog)` marker if it overflows.

Test plan

• `TestCheck_CapturesReleaseBody` — GH response body flows into `Info.ReleaseBody`.
• `TestCheck_TruncatesHugeReleaseBody` — > 16 KiB body gets truncated with marker.
• `make test` + `make e2e` — green.
• Smoke on Pi: next release lands, modal shows the "### Bug Fixes / ### Features" sections rendered as headings + bullets, links clickable.

🤖 Generated with Claude Code