T4.1: Rename gui->ui, split lifecycle into run.go + mainwindow.go

Carve src/gui/app.go into the new src/ui package:
- run.go: process lifecycle (single instance, app/window, tray, startup
  timing).
- mainwindow.go: view assembly + the app.Service event listener.

Route every widget update driven by Service events through fyne.Do so the
run goroutine (executeRun) no longer mutates Fyne widgets directly. Also
wrap serveSingleInstance's Show/RequestFocus, which runs on the Accept
goroutine. (Resolves refactoring problem #4.)

fyne.Do/DoAndWait only exist in Fyne v2.6+, so upgrade fyne.io/fyne/v2
v2.5.3 -> v2.6.3. Mark the tray Quit item IsQuit so Fyne's
addMissingQuitForMenu reuses it instead of appending a second, localized
Quit now that v2.6 ships Russian translations.

go build / go vet / go test -race all clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

cmd/gosentry/main.go

@@ -4,14 +4,14 @@ import (
 	"os"
 
 	"gitea.mixdep.ru/mix/gosentry/src/domain"
-	"gitea.mixdep.ru/mix/gosentry/src/gui"
+	"gitea.mixdep.ru/mix/gosentry/src/ui"
 )
 
 func main() {
 	// The executable entry point intentionally delegates all startup work to the
-	// GUI package. Keeping main small makes it easier to add platform-specific
+	// UI package. Keeping main small makes it easier to add platform-specific
 	// packaging later without mixing window setup, storage, and scheduler logic.
-	gui.Run(hasArgument(domain.StartInTrayArgument))
+	ui.Run(hasArgument(domain.StartInTrayArgument))
 }
 
 func hasArgument(argument string) bool {