feat(ui): render release notes as markdown in the update dialog (#132)

The 'new release available' dialog showed GitHub release notes as raw
whitespace-pre text. Render them with react-markdown + remark-gfm via a
new styled, XSS-safe Markdown component (no rehype-raw; links forced to
target=_blank rel=noopener; images constrained). Covers headings, lists,
links, inline/fenced code, tables, strikethrough, blockquotes, task-list
checkboxes and images, all styled to the dark UI.

Author: christianromeni