macOS: first-run onboarding notification

[hvtim]

Companion to #31 (Windows balloon tip) - extending the same first-run onboarding fix to macOS for v0.1.2 rather than shipping it Windows-only.

StatusItemTray/StatusItemTray.mm already imports <Cocoa/Cocoa.h> (Foundation transitively). NSUserNotificationCenter/NSUserNotification is deprecated but still present with zero new frameworks/linker flags - the modern UNUserNotificationCenter replacement needs a proper bundle identifier + entitlement/framework link, i.e. a real new dependency, so it's not the minimal-footprint choice here. The app is already MACOSX_BUNDLE with an Info.plist (CMakeLists.txt:137), so NSUserNotificationCenter defaultUserNotificationCenter with a title/informativeText notification is the right fit.

Same trigger condition as #31: on startup, if config.clientId is still the placeholder default (core/AppConfig.h), show one notification once per launch: "FeatherRPC is running. Right-click the tray icon to set your Discord Application ID."

Tasks

• Add a method to native/src/platform/macos/StatusItemTray.h/.mm that posts an NSUserNotification via NSUserNotificationCenter
• Call it once from native/src/platform/macos/main.mm after tray creation, gated on the placeholder client ID check
• Build and run on the macOS Intel test VM to confirm the notification appears with a placeholder client ID, and does NOT appear after appid set

Part of the v0.1.2 onboarding UX pass. See also #31 (Windows), Linux companion issue.

[hvtim]

PR: #44. Code path verified correct via system logs end-to-end; the visual banner itself is gated behind a one-time macOS permission prompt nobody's clicked through on this headless VM - same category of limitation as the Windows/Linux companions. Worth a quick manual check on an interactive session.

[hvtim]

Update: #55 (open, CI green, mergeable) resolves this, but via a different design than the checklist above describes. NSUserNotificationCenter turned out not to be viable: its first-ever call gates behind a Big Sur+ permission prompt, and that first notification is typically dropped even if allowed - no reliable way to guarantee it appears on a genuinely fresh launch. The modern UNUserNotificationCenter doesn't help either, since it requires the app to be code-signed (#29) before that permission dialog even appears. #55 replaces the notification with StatusItemTray::ShowFirstRunMenu(), which opens the tray's own dropdown menu directly instead - no OS permission gate either way, and it stays open until dismissed rather than auto-disappearing. Will auto-close when #55 merges.