v0.2.2: device-sync guards — quit warning, sleep/shutdown inhibition

Quitting mid-transfer now asks (Keep Syncing / Quit Anyway) from
closeEvent, which every quit path hits — including the self-update
restart, which previously bypassed closeEvent via a bare quit() and now
routes through close(). A running sync holds its own GNOME inhibitor
(suspend + logout, "Syncing a playlist to a device") so the machine
won't sleep or shutdown/restart under a copy.

Along the way: the Inhibit D-Bus call marshaled Python ints as signed
against GNOME's (susu) signature, so every call was rejected and the
playback sleep inhibitor had silently never worked — _uint fixes both
holders (verified live: cookies taken, IsInhibited flips, releases
clean).

Also investigated trav's mid-copy Syncthing question: the transfer works
from a click-time snapshot and never reads live library state, so remote
changes can't corrupt it — no state freeze needed. Hardened the one real
gap: a source file relocated under the queue (remote metadata edit
moving files) is now skipped, dropped from the m3u, and reported,
instead of aborting the whole sync.

tests/test_round26.py

@@ -177,15 +177,16 @@ class TestWorker:
         assert seen[-1][0] == seen[-1][1] == 2 * device_sync.CHUNK // 1024
         assert "1/1" in seen[-1][2]
 
-    def test_source_vanishing_reports_failure(self, qapp, tmp_path):
+    def test_device_side_error_reports_failure(self, qapp, tmp_path):
+        # A file squatting on the dest-dir path stands in for a device-side
+        # write error (unplug mid-sync). Vanished *sources* are tolerated
+        # (round 27); device errors still abort.
         a = _audio(tmp_path, "a.mp3")
         plan = plan_sync("Mix", [_track(1, "A", "X", a)], tmp_path / "Music")
-        a.unlink()  # unplugged/deleted between plan and run
+        (tmp_path / "Music").mkdir()
+        (tmp_path / "Music" / "Mix").write_bytes(b"not a dir")
         result = self._synced(qapp, plan)
         assert "Sync failed" in result["error"]
-        # No half-copied file left behind at full-looking size.
-        leftover = plan.dest_dir / "X - A.mp3"
-        assert not leftover.exists() or leftover.stat().st_size != plan.copies[0].size
 
     def test_empty_playlist_clears_folder(self, qapp, tmp_path):
         dest = tmp_path / "Music" / "Mix"