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/web/src/platform/electron.d.ts

@@ -42,6 +42,11 @@ interface BackspaceElectronAPI {
   getInstanceUrl: () => Promise<string | null>;
   setInstanceUrl: (url: string) => Promise<void>;
   clearInstanceUrl: () => Promise<void>;
+
+  // Auto-launch settings
+  getAutoLaunchSettings: () => Promise<{ openAtLogin: boolean; startMinimized: boolean }>;
+  setAutoLaunchSettings: (settings: { openAtLogin?: boolean; startMinimized?: boolean }) =>
+    Promise<{ openAtLogin: boolean; startMinimized: boolean }>;
 }
 
 interface Window {