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>

scripts/build-deb.sh

@@ -16,10 +16,17 @@ case $kind in
 client)
 	pkg=keeporg
 	binary=keeporg
-	mkdir -p "$stage/usr/bin" "$stage/usr/lib/systemd/user"
+	mkdir -p "$stage/usr/bin" "$stage/usr/lib/systemd/user" \
+		"$stage/usr/share/applications" "$stage/usr/share/icons"
 	install -m 0755 "$root/build/linux-$arch/keeporg" "$stage/usr/bin/keeporg"
 	install -m 0644 "$root/packaging/systemd/keeporg.service" \
 		"$stage/usr/lib/systemd/user/keeporg.service"
+	# The tray application: a menu entry, and the icon it and the tray use.
+	install -m 0644 "$root/packaging/linux/keeporg.desktop" \
+		"$stage/usr/share/applications/keeporg.desktop"
+	cp -R "$root/build/icons/hicolor" "$stage/usr/share/icons/hicolor"
+	rm -f "$stage/usr/share/icons/hicolor/.stamp"
+	find "$stage/usr/share/icons" -type d -exec chmod 0755 {} + -o -type f -exec chmod 0644 {} +
 	;;
 server)
 	pkg=keeporgd