Linux now shows monochrome (black/white silhouette) tray icons instead
of the colored orange PNGs, matching the macOS template look. Since
Wails' Linux SNI backend ignores SetDarkModeIcon (its setDarkModeIcon
just calls setIcon, last-write-wins) and the SNI spec carries no panel
light/dark hint, the panel color scheme is detected in-process and the
black-vs-white silhouette is chosen in iconForState, pushed via a single
SetIcon.
Detection order (tray_theme_linux.go): freedesktop Settings portal
(org.freedesktop.appearance/color-scheme) -> GTK_THEME env (:dark
suffix) -> default dark. A SettingChanged subscription repaints live on
theme flips. macOS (template) and Windows (colored) paths are unchanged.
Icons are 48x48 mono PNGs (3% margin) generated from the macOS
silhouettes.
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