T4.6: Extract tray.go, singleinstance.go, layout.go

Extract three focused modules from run.go and mainwindow.go:

- tray.go: configureSystemTray() and related tray setup
- singleinstance.go: acquireSingleInstance(), serveSingleInstance(), and constants
- layout.go: minWidthLayout type and methods

No behavior change; purely mechanical extraction to improve file organization.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

src/ui/tray.go

@@ -0,0 +1,40 @@
+package ui
+
+import (
+	"fyne.io/fyne/v2"
+	fynedesktop "fyne.io/fyne/v2/driver/desktop"
+)
+
+func configureSystemTray(a fyne.App, w fyne.Window) {
+	desk, ok := a.(fynedesktop.App)
+	if !ok {
+		// Not every Fyne driver exposes desktop tray features. Returning silently
+		// keeps the same binary usable on platforms or sessions without a tray.
+		return
+	}
+
+	// IsQuit marks this as the tray's quit item. Without it Fyne's
+	// addMissingQuitForMenu appends a second, localized Quit (e.g. "Выход" on a
+	// Russian system) because it only recognizes an existing quit by matching the
+	// localized label — which our literal "Quit" does not. Setting IsQuit makes
+	// Fyne reuse this item instead of adding a duplicate, regardless of locale.
+	quit := fyne.NewMenuItem("Quit", func() {
+		a.Quit()
+	})
+	quit.IsQuit = true
+	menu := fyne.NewMenu("GoSentry",
+		fyne.NewMenuItem("Show", func() {
+			w.Show()
+			w.RequestFocus()
+		}),
+		fyne.NewMenuItemSeparator(),
+		quit,
+	)
+	desk.SetSystemTrayMenu(menu)
+	w.SetCloseIntercept(func() {
+		// Closing hides the window instead of quitting because scheduler tools are
+		// expected to keep working in the background. The explicit Quit tray item
+		// remains the way to stop the process.
+		w.Hide()
+	})
+}