Split Linux panel-theme detection into two files and fix the KDE case
where the tray icon picked the wrong mono variant.
The freedesktop Settings portal's color-scheme reports the *global*
light/dark preference, but the KDE panel is painted from the
Complementary colour group, which can be dark even when the global
scheme is Light. The tray sits on the panel, so keying its black/white
mono icon off the portal value alone gave the wrong contrast on KDE.
Changes:
- tray_theme_linux.go keeps the dark/light decision; on KDE it now
reads the user's kdeglobals [Colors:Complementary] BackgroundNormal
to determine the actual panel luminance, falling back to the portal
color-scheme / GTK_THEME chain elsewhere.
- tray_theme_watcher_linux.go (new) owns the live half: a private
session-bus connection for the portal SettingChanged signal plus an
fsnotify watch on kdeglobals, repainting the tray on a panel-theme
flip.
- tray_theme_linux_test.go (new) covers the kdeglobals Complementary
parse against the KDE test-VM's real file layout.
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.
