refactor: collapse micro-modules, add AUR packaging

- domain/ (7 files) → domain.rs + error.rs + window.rs
- protocols/sftp/ (4 files) → sftp.rs, ftp/ (3 files) → ftp.rs
- storage/db/ (6 files) → storage.rs + settings.rs
- transfer/ (5 files) → transfers/{queue,worker}.rs
- add dist/PKGBUILD, .SRCINFO, wherry.desktop for AUR
- fix all import paths, 53 tests pass

dist/wherry.desktop

@@ -0,0 +1,9 @@
+[Desktop Entry]
+Type=Application
+Name=Wherry
+Comment=FTP/SFTP client with dual-pane file manager
+Exec=wherry
+Icon=wherry
+Categories=Network;FileTransfer;Utility;
+Terminal=false
+StartupNotify=true