Turn the desktop client into a system tray application

Started without a command, keeporg now shows a padlock in the system tray
with a menu like midi-webhook's: a status line, the mirror folder, each
database with its state, Sync Now, browser sign-in and sign-out, the web
interface, the configuration, the log, Start at Login, About and Quit. The
commands are unchanged.

config.json now carries every client setting (device_label,
request_timeout_seconds, ca_file, log_file, notifications), is written in
full on first start and is reloaded by the tray when saved.

A lock in the mirror directory admits one agent at a time, since the tray,
`keeporg run` and a service unit could otherwise race each other.

The tray uses fyne.io/systray: pure Go on Linux, BSD and Windows, cgo on
macOS only. The server keeps no third-party dependencies.

Desktop versions go to 0.5.0.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packaging/linux/keeporg.desktop

@@ -0,0 +1,11 @@
+[Desktop Entry]
+Type=Application
+Name=KeepOrg
+GenericName=Password database synchronization
+Comment=Keep shared KeePass databases in step with a KeepOrg server
+Exec=keeporg tray
+Icon=keeporg
+Terminal=false
+Categories=Utility;Security;
+Keywords=keepass;kdbx;password;sync;
+StartupNotify=false