fix(download): honor retry and queue limits

Treat max_tries as the configured retry count instead of total attempts across native, media, aria2, and fallback paths.

Route queue slot acquisition through retirement-aware permit handling so shrinking concurrency does not dispatch new work while active transfers still meet the smaller limit.

Update retry-budget and queue-manager tests to cover the corrected attempt counts and paused aria2 resume behavior.

src-tauri/tests/download_engine.rs

@@ -427,7 +427,7 @@ async fn reports_terminal_http_errors_after_retry_budget() {
     };
 
     assert!(error.contains("500 Internal Server Error"));
-    assert_eq!(server.state.requests.load(Ordering::SeqCst), 2);
+    assert_eq!(server.state.requests.load(Ordering::SeqCst), 3);
     assert!(!output_path.exists());
 }