Improve crash diagnostics and isolate watcher-thread failures

Main-thread unhandled exceptions now show the traceback and log path; worker-thread exceptions log without touching Qt widgets. The download watcher catches its own loop failures and reports that it stopped instead of surfacing a blank error dialog. Adds a regression test for the watcher failure path.

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

tests/test_realtime.py

@@ -58,6 +58,19 @@ def test_firefox_download_preference_is_watched(tmp_path, monkeypatch):
     assert download.resolve() in [path.resolve() for path in roots]
 
 
+def test_watcher_stops_cleanly_instead_of_raising(monkeypatch, qapp):
+    messages: list[str] = []
+    watcher = realtime.DownloadWatcher(interval=0.1)
+    watcher.watch_status.connect(messages.append)
+    monkeypatch.setattr(realtime, "browser_download_roots", lambda extra: (_ for _ in ()).throw(
+        RuntimeError("simulated watcher failure")))
+    watcher.start()
+    assert watcher.wait(3000)
+    qapp.processEvents()
+    assert not watcher.isRunning()
+    assert "stopped" in " ".join(messages).lower()
+
+
 def test_copy_for_scan_preserves_bytes(tmp_path):
     source = tmp_path / "download.exe"
     source.write_bytes(b"sample bytes")