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>

versions.mk

@@ -1,10 +1,10 @@
 # Single source of truth for all platform versions (semver).
 # Bump with: make bump PLATFORM=<server|linux|macos|windows|bsd|android> PART=<major|minor|patch>
 VERSION_SERVER   := 0.7.0
-VERSION_LINUX    := 0.4.0
-VERSION_MACOS    := 0.4.0
-VERSION_WINDOWS  := 0.4.0
-VERSION_BSD      := 0.4.0
+VERSION_LINUX    := 0.5.0
+VERSION_MACOS    := 0.5.0
+VERSION_WINDOWS  := 0.5.0
+VERSION_BSD      := 0.5.0
 VERSION_ANDROID  := 0.2.0
 # Monotonic Android versionCode, never decreases.
 ANDROID_VERSION_CODE := 2