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>

tools/genicon/main.go

@@ -0,0 +1,46 @@
+// Command genicon writes the application icon for packaging.
+//
+//	genicon -hicolor <dir>   <dir>/<n>x<n>/apps/keeporg.png, the Linux icon theme layout
+//	genicon -iconset <dir>   <dir>/icon_<n>x<n>[@2x].png, the input iconutil turns into .icns
+package main
+
+import (
+	"flag"
+	"fmt"
+	"os"
+	"path/filepath"
+
+	"cc.tiredsysadmin.keeporg/internal/icon"
+)
+
+func main() {
+	hicolor := flag.String("hicolor", "", "write a hicolor icon theme tree under this directory")
+	iconset := flag.String("iconset", "", "write a macOS .iconset directory")
+	flag.Parse()
+	if *hicolor == "" && *iconset == "" {
+		flag.Usage()
+		os.Exit(2)
+	}
+	if *hicolor != "" {
+		for _, n := range []int{16, 32, 48, 64, 128, 256, 512} {
+			write(filepath.Join(*hicolor, fmt.Sprintf("%dx%d", n, n), "apps", "keeporg.png"), n)
+		}
+	}
+	if *iconset != "" {
+		for _, n := range []int{16, 32, 128, 256, 512} {
+			write(filepath.Join(*iconset, fmt.Sprintf("icon_%dx%d.png", n, n)), n)
+			write(filepath.Join(*iconset, fmt.Sprintf("icon_%dx%d@2x.png", n, n)), 2*n)
+		}
+	}
+}
+
+func write(path string, size int) {
+	if err := os.MkdirAll(filepath.Dir(path), 0o755); err != nil {
+		fmt.Fprintln(os.Stderr, "genicon:", err)
+		os.Exit(1)
+	}
+	if err := os.WriteFile(path, icon.PNG(icon.App(size)), 0o644); err != nil {
+		fmt.Fprintln(os.Stderr, "genicon:", err)
+		os.Exit(1)
+	}
+}