Unify ProjectDetailView and WorktreeDetailView (~75% code overlap)

[2witstudios]

Problem

ProjectDetailView.swift (192 lines) and WorktreeDetailView.swift (168 lines) share ~75% of their code:

• Identical overall structure: VStack { header; Divider; tabBar; Divider; content }
• Identical tab bar (segmented picker for unstaged/PR diffs, same padding)
• Identical unstaged content section (delegates to DiffListView)
• Nearly identical PR content section
• Identical ghUnavailableView (already extracted to shared GHUnavailableView in recent cleanup)
• Same @State private var diffState = DiffState() pattern
• Same .task / .onDisappear lifecycle

The only real differences are:

1. The header shows project info vs worktree info (icon, name, metadata)
2. PR selection UI (list with back-navigation vs dropdown picker)
3. The data loading call (loadForProject vs loadForWorktree)

Suggested approach

Create a single DiffDetailView parameterized on the entity, with the header and PR selection UI injected via @ViewBuilder or a protocol. Estimated ~100 lines saved.

Files

• apps/purepoint-macos/purepoint-macos/Views/Detail/ProjectDetailView.swift
• apps/purepoint-macos/purepoint-macos/Views/Detail/WorktreeDetailView.swift