refactor: make subsync manual-only, default opt-in features off, preserv

- Remove subsync.defaultMode; subsync always opens manual picker
- Default jellyfinRemoteSession warmup and nameMatchEnabled to false
- Stop rewriting config file during legacy migration (resolve in-memory only)
- Fix macOS quit on window-close for --setup launch mode

changes/config-load-preserves-user-files.md

@@ -0,0 +1,4 @@
+type: fixed
+area: config
+
+- Preserved user config files during legacy config compatibility handling.

changes/default-opt-in-warmup-name-highlighting.md

@@ -0,0 +1,4 @@
+type: changed
+area: config
+
+- Defaulted Jellyfin remote-session startup warmup and character-name subtitle highlighting to off.

changes/subsync-manual-only.md

@@ -0,0 +1,4 @@
+type: changed
+area: subtitles
+
+- Subsync now always opens the manual picker and the `subsync.defaultMode` config/settings option has been removed.