UI update: clearer Xcode hierarchy, tags and Settings

[anciltech]

The Xcode sidebar repeats information across groups and makes release type, active selection, and installation progress difficult to scan. This update presents one major/minor hierarchy with clearer labels and controls, and adds appearance settings that work in both grouped and flat lists.

Changes

• Distinguish major, minor, and version rows with tree guides, counts, and readable selection styling. Keep version titles visible; put build identifiers and install paths on the secondary line.
• Show Beta, RC, Latest, and Active tags, including in the flat list. Expanded groups leave Open/Install, checkmarks, and progress on the deepest visible row.
• Replace repeated progress text with a compact progress/cancel ring; keep detailed progress in the info pane and tooltip.
• Put Collapse Major/Minor versions and Show Tags in a UI section above Misc. Grouping defaults to enabled for new users while preserving existing preferences. Replace the older parallel grouping implementation with the current hierarchy.
• Correct Settings tab tinting and give the sidebar more room. Include the updater observation warning fix in the Settings code touched here.

Validation

• Full macOS unit suite: 54 tests passed with Xcode 27.1 beta, Test configuration, arm64 destination, signing disabled.
• Manually checked Settings tab selection and live tag toggling with grouping enabled and disabled.
• Combined with the other two topics: all 75 tests passed.

Reviewer checks

1. Expand/collapse a major and minor group; verify the active version remains clear and controls do not repeat.
2. Turn collapsing off. Toggle Show Tags and verify Beta/RC names remain identifiable with tags hidden.
3. Switch Settings tabs and verify only the selected tab has the accent color.

Consolidates #751 and #865. Related independent topics: Usability improvements and Platform management. The combined preview branch contains the complete app and tested resolutions for the shared code.

Related review topics: #874, #875.

Related issues

• No progress bar visible during the installation of Xcode. #772 — adds inline installation progress and cancellation beside the affected Xcode.
• Dancing Icons in Settings #717 — replaces the Settings tab-bar presentation behind the reported unstable icon layout.
• Clicking install button doesnt seem as anything is happening #757, Xcodes 3.0.2(34) doesn't show progress updates or installed state #769, and Xcode 26.3 download stuck indefinitely at "Step 2 of 7: Downloading" #846 — make installation progress much more visible. This does not add stalled-download detection or retry reporting.