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/deb/client/control.in

@@ -3,6 +3,8 @@ Version: @VERSION@
 Section: utils
 Priority: optional
 Architecture: @ARCH@
+Recommends: libnotify-bin
+Suggests: keepassxc, zenity | kdialog
 Maintainer: tiredsysadmin <rcmd@garage.rcmd.space>
 Homepage: https://garage.rcmd.space/keeporg
 Description: KeePass database synchronization client
@@ -13,5 +15,10 @@ Description: KeePass database synchronization client
  Each save is pushed to the server and changes other people push are applied
  locally. The client never opens a database and never sees a master password.
  .
- A systemd user unit is included: enable it with
- `systemctl --user enable --now keeporg`.
+ KeepOrg starts as a system tray application from the applications menu; its
+ menu signs in, shows each database's state and opens the settings. The tray
+ needs a StatusNotifierItem host: KDE, XFCE, Cinnamon and most panels have
+ one, GNOME needs the AppIndicator extension.
+ .
+ Without a desktop, `keeporg run` is the same agent, and a systemd user unit
+ is included for it: `systemctl --user enable --now keeporg`.