feat: add start-at-boot setting and fix tray icon to use app logo

Add auto-launch settings (start at boot + start minimized) to the
Desktop section in Account settings, using Electron's built-in
setLoginItemSettings API across macOS, Windows, and Linux.

Replace the fallback colored-circle tray icon with proper B logo assets:
template images for macOS (adapts to light/dark menu bar) and colored
icons for Windows/Linux. Fix BGRA channel order bug in fallback generator
and move tray icons to resources/ so they're packaged into the app.

packages/desktop/src/preload.ts

@@ -62,4 +62,9 @@ contextBridge.exposeInMainWorld('backspace', {
   getInstanceUrl: () => ipcRenderer.invoke('get-instance-url'),
   setInstanceUrl: (url: string) => ipcRenderer.invoke('set-instance-url', url),
   clearInstanceUrl: () => ipcRenderer.invoke('clear-instance-url'),
+
+  // Auto-launch settings
+  getAutoLaunchSettings: () => ipcRenderer.invoke('get-auto-launch-settings'),
+  setAutoLaunchSettings: (settings: { openAtLogin?: boolean; startMinimized?: boolean }) =>
+    ipcRenderer.invoke('set-auto-launch-settings', settings),
 });