fix(push-notifications): avoid duplicate macOS notification icons

[smsunarto]

Human comments

What was wrong

Desktop notifications always supplied /icon-192.png through the Web Notification API. On macOS, Electron displays this as a content image alongside the app icon macOS already adds, producing two BB logos.

What changed

Omit the explicit notification icon when the desktop bridge identifies macOS. Keep the existing icon for web and Linux desktop notifications. Notification content, navigation, and deduplication remain unchanged.

How you verified

• Added macOS, Linux, and web regression cases. Before the fix, only the macOS case failed because the extra icon was present. After the fix, all 20 plugin tests pass.
• NODE_OPTIONS=--no-experimental-webstorage pnpm exec turbo run test typecheck --filter=bb-plugin-push-notifications passes on Node 22.23.2. Disabling Node's experimental web storage lets jsdom own localStorage.
• Formatting and oxlint pass for both changed files.
• Built and reloaded the plugin in the source-built Electron app, then triggered the real notifications.test RPC. Observed Electron's Notification constructor receiving the title, body, and tag with no icon. Native delivery remained enabled during observation. Desktop logs showed no notification errors.

Before and after

The after capture uses the source-built dev app, so the single application icon is Electron's development icon.

Before: duplicate icons	After: single application icon

AGENT GENERATED