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/electron-builder.yml

@@ -9,7 +9,7 @@ files:
   - "!node_modules"
 publish:
   - provider: generic
-    url: "${BACKSPACE_UPDATE_URL}"
+    url: "${env.BACKSPACE_UPDATE_URL}"
     useMultipleRangeRequest: false
 protocols:
   - name: Backspace
@@ -24,6 +24,7 @@ mac:
     - dmg
     - zip
 win:
+  icon: ./build/icon.ico
   target:
     - nsis
 linux: