Decode portal wallpapers by content and simplify status bar

Detect wallpaper image formats from their contents so extensionless portal copies render on the desktop and lock screen. Preserve decode limits and cover extensionless files with regression tests.

Remove the Bluetooth icon from the status bar.

shell/ferese-shell/src/status_ui.rs

@@ -252,11 +252,10 @@ impl FereseShell {
     pub fn view_status_bar(&self) -> Element<'_, cosmic::Action<Message>> {
         let theme = self.config.theme.for_bar();
         let metrics = BarMetrics::from(theme);
-        let mut controls = row::with_capacity(7).spacing(1).align_y(Alignment::Center);
+        let mut controls = row::with_capacity(6).spacing(1).align_y(Alignment::Center);
         for kind in [
             Menu::System,
             Menu::Network,
-            Menu::Bluetooth,
             Menu::Audio,
             Menu::Recording,
             Menu::Notifications,