* Add dark mode app icon variant for macOS Sequoia
Adds dark appearance entries to the AppIcon asset catalog so macOS 15+
automatically shows a dark-background icon when the system is in dark
mode. The chevron gradient and glow are preserved by recompositing the
foreground over a dark background (#1C1C1E).
Includes a generation script (scripts/generate_dark_icon.py) that
derives the dark PNGs from the light originals.
* Add icon picker in Settings and fix dark icon quality
Use the Figma chevron layer (design/cmux-icon-chevron.png) composited
over a dark background for pixel-perfect results, no white halo or
darkened gradient. Falls back to mathematical recomposition if the Figma
layer is missing.
Add an "App Icon" picker to Settings (under Theme) with three visual
options: Automatic (follows system appearance via asset catalog dark
variants on macOS 15+), Light, and Dark. The selection persists via
UserDefaults and is applied on launch in AppDelegate.ensureApplicationIcon.
* Fix dark icon chevron scale to match light icon
The Figma export was ~25% larger than the repo icon. Scale the Figma
chevron layer by 0.80x before compositing so the chevron size matches
exactly between light and dark variants.
* Use enhanced glow for dark icon
Add a soft blue bloom around the chevron on the dark background using
two Gaussian blur passes (wide at r=25 and tight at r=12) composited
at reduced opacity beneath the sharp chevron. Makes the icon pop more
against the dark squircle.
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