Quit guard during transfers + background completion notifications

- before-quit is blocked while jobs are queued/scanning/running; a native
  message box asks 'N transfers are still running - quit anyway?' (Cancel
  default) and only an explicit Quit Anyway proceeds via Utils.quit()
- when the window is unfocused, finished transfers raise a native
  notification (silent for completions, sound + error message for failures)
- engine.hasActiveJobs()/activeJobCount() (truth table verified headless)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/bun/transfers/engine.ts

@@ -146,6 +146,22 @@ export class TransferEngine {
 		if (job && TERMINAL.includes(job.snapshot.state)) this.jobs.delete(jobId);
 	}
 
+	/** True while any job is queued, scanning or running (quit guard). */
+	hasActiveJobs(): boolean {
+		for (const job of this.jobs.values()) {
+			if (!TERMINAL.includes(job.snapshot.state)) return true;
+		}
+		return false;
+	}
+
+	activeJobCount(): number {
+		let count = 0;
+		for (const job of this.jobs.values()) {
+			if (!TERMINAL.includes(job.snapshot.state)) count++;
+		}
+		return count;
+	}
+
 	/** Fail every non-terminal job that touches a dead session. */
 	failSession(sessionId: string, reason: string): void {
 		for (const job of this.jobs.values()) {