* Add tab color feature to sidebar workspaces
Lets users assign a custom background color to any sidebar workspace tab
via a right-click "Tab Color" submenu. The primary motivation is working
across multiple projects simultaneously — coloring tabs by project makes
it instant to visually locate the right workspace without reading the title.
- Workspace: adds `customColor: String?` (@Published hex string) and
`setCustomColor()` setter
- TabManager: adds `setTabColor(tabId:color:)` convenience method
- ContentView: 16-color dark palette (all luminance < 0.30, white text
always readable), `Color(hex:)` extension, `coloredCircleImage(hex:)`
helper to render bitmapped NSImage circles (needed because macOS menus
strip SwiftUI foregroundColor from SF Symbols), updated `backgroundColor`
to use custom color at full/70%/35% opacity for active/inactive/
multi-selected states, "Tab Color" submenu in context menu with
"Clear Color" option, and a 1.5pt `Color.primary` border overlay on
the active tab for clear selection indication when custom colors are set
* Add workspace tab color schemes with settings and debug toggles
* Remove Kelly scheme and keep only original tab color palette
* Preserve neutral grayscale when brightening tab colors
* Harden UpdatePill UI test polling timeouts
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Co-authored-by: Andreas Fruth <andreas.fruth@gmail.com>
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