fix(updater): show complete release notes

[sophiathedev]

Summary

Make it clear what an offered update changes, directly in the update window.

• Embed the complete version-specific changelog as native Sparkle Markdown, with new features before improvements and fixes. Keep all other sections and make long notes scrollable.
• Use the same notes for Sparkle and GitHub Releases. Reject missing notes instead of publishing a generic fallback.
• Add What's New to Help and Software Update settings, plus a full-changelog link in Sparkle.

The existing feed already contains release notes. The confirmed rendering problem was the hand-built HTML leaving Markdown visible and treating literal angle brackets as HTML; this does not claim to reproduce an empty update window.

Validation

• 11 release-note regression tests passed, including exact-version matching, section ordering, fenced Markdown, and complete preservation of the latest release.
• App and test bundles built; all 8 MainMenuStructureTests passed.
• Strict SwiftLint: 0 violations in the changed Swift files. Bash syntax, workflow YAML parsing, and diff checks passed.
• Ran the actual signing/appcast script with Sparkle 2.9.5 and temporary test keys: both architecture entries were signed, notes were complete, and existing feed history survived.
• Verified all 231 changes from 0.73.0 in Sparkle's native text view, with features before fixes and a working scrollable document.

Evidence

Actual Sparkle UI in a clearly labeled test app with TablePro's bundled icon, using the real 0.73.0 changelog and a localhost feed. No update was installed.

Top: new features first

Bottom: final changes and full history link

Screenshots are pinned to a separate evidence commit in the fork; no image files are added to this PR's code diff. The published appcast is unchanged; the new formatting applies when a release is generated with this pipeline.