Small UI improvements: detail toolbar, loading overlay, empty states (#258)
* Align empty-state layouts with detail placeholder
* Normalize sidebar icon sizing and repo add button
* Redesign worktree loading overlay
Match the detail-placeholder layout (large spinner, title, caption,
5-line animated tail) and replace the flat `state: .creating` /
`statusLines:` bag with a `WorktreeLoadingInfo.Kind` enum carrying
a `Progress` payload. Dead `.archiving` case dropped since archiving
routes via the terminal.
* Redesign detail title with avatar and folder support
Replace the plain branch label with a Label-based button carrying the
GitHub owner avatar (same layout used in Settings), a rename popover
for git worktrees, and a disabled folder-glyph label for folder
repositories. `WorktreeToolbarState` now carries a `Kind` enum so
"folder with a pull request" is unrepresentable.
* Inline notification bell with status toolbar item
* Hide window-toolbar background and center placeholder text
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