fix: polish update screen release notes UX

[Regstar2]

Polishes the Issue #6 update-delivery screen after manual Android review.

Changes:

• keeps the installed-version card compact;
• moves the official-source/security explanations to secondary text below the main controls;
• places Open official release immediately in the available-update card;
• converts GitHub Markdown release notes to readable plain text instead of showing raw #, **, backticks and Markdown links;
• shows only a short release-notes preview by default;
• adds Show full release notes / Hide full release notes controls;
• keeps Check for updates as a separate compact action;
• adds RU/EN strings for the new controls;
• adds unit tests for Markdown cleanup, fenced-code handling, preview truncation and short-note behavior.

No update-selection, release-source, permissions, proxy runtime, or networking behavior is changed.

Refs #6