chore(wailsv3): migrate Go backend from Wails v2 to v3

Minimal-structure migration to Wails v3.0.0-beta.20 (CLI-matching pin). Single App service preserved; no domain logic touched.

- main.go: wails.Run(options.App) -> application.New + RegisterService + Window.NewWithOptions. Window keeps title-with-version, 1024x768, RGBA background, Linux icon/translucency/GpuPolicyNever, ProgramName. SingleInstanceLock -> SingleInstanceOptions (same UniqueID); callback closes over the service variable set right after New.
- app.go: App holds *application.App instead of a context. startup(ctx) is gone; the version title moves into the window options via appTitle(). onSecondInstanceLaunch takes v3 SecondInstanceData (Args/WorkingDir) and uses Window.Current() Restore+Focus plus Event.Emit for launchArgs. ShowVersion uses Dialog.Info.
- AniList/MAL/Simkl *UserFunctions.go: runtime.BrowserOpenURL -> app.Browser.OpenURL (errors logged), runtime.MessageDialog -> app.Dialog.Info chains. OAuth callback servers, token flows, and key names unchanged.
- Secrets: 99designs/keyring replaced with zalando/go-keyring (what v3 itself depends on) under the same AniTrack service name, so existing stored tokens keep working. Per-file Set helpers kept; Open/Ready boilerplate deleted (zalando needs no handle).
- go.mod: v2 + 99designs dropped (stale v2 replace directive removed), v3 beta.20 + zalando v0.2.8 added.

main.go

@@ -2,45 +2,56 @@ package main
 
 import (
 	"embed"
+	"log"
 
-	"github.com/wailsapp/wails/v2"
-	"github.com/wailsapp/wails/v2/pkg/options"
-	"github.com/wailsapp/wails/v2/pkg/options/assetserver"
-	"github.com/wailsapp/wails/v2/pkg/options/linux"
+	"github.com/wailsapp/wails/v3/pkg/application"
 )
 
 //go:embed all:frontend/dist
 var assets embed.FS
 
 func main() {
-	// Create an instance of the app structure
-	app := NewApp()
+	// The service is created after the application (it needs the app
+	// reference), so the single-instance callback closes over the variable
+	// and resolves it at call time. A second instance can only launch once
+	// this one is running, by which point svc is always set.
+	var svc *App
 
 	// Create application with options
-	err := wails.Run(&options.App{
-		Title:  "AniTrack",
-		Width:  1024,
-		Height: 768,
-		AssetServer: &assetserver.Options{
-			Assets: assets,
+	app := application.New(application.Options{
+		Name:        "AniTrack",
+		Description: "Track anime watchlists across AniList, MyAnimeList, and Simkl",
+		Assets: application.AssetOptions{
+			Handler: application.AssetFileServerFS(assets),
 		},
-		BackgroundColour: &options.RGBA{R: 27, G: 38, B: 54, A: 1},
-		OnStartup:        app.startup,
-		SingleInstanceLock: &options.SingleInstanceLock{
-			UniqueId:               "49c93b6d-663d-4b7a-9cb0-8a469ea9182b",
-			OnSecondInstanceLaunch: app.onSecondInstanceLaunch,
+		SingleInstance: &application.SingleInstanceOptions{
+			UniqueID: "49c93b6d-663d-4b7a-9cb0-8a469ea9182b",
+			OnSecondInstanceLaunch: func(data application.SecondInstanceData) {
+				svc.onSecondInstanceLaunch(data)
+			},
 		},
-		Bind: []interface{}{
-			app,
-		},
-		Linux: &linux.Options{
-			Icon:                []byte("./build/AniTrack.png"),
-			WindowIsTranslucent: false,
-			WebviewGpuPolicy:    linux.WebviewGpuPolicyNever,
-			ProgramName:         "AniTrack",
+		Linux: application.LinuxOptions{
+			ProgramName: "AniTrack",
 		},
 	})
-	if err != nil {
-		println("Error:", err.Error())
+
+	svc = NewApp(app)
+	app.RegisterService(application.NewService(svc))
+
+	app.Window.NewWithOptions(application.WebviewWindowOptions{
+		Title:            appTitle(),
+		Width:            1024,
+		Height:           768,
+		BackgroundColour: application.NewRGBA(27, 38, 54, 1),
+		URL:              "/",
+		Linux: application.LinuxWindow{
+			Icon:                []byte("./build/AniTrack.png"),
+			WindowIsTranslucent: false,
+			WebviewGpuPolicy:    application.WebviewGpuPolicyNever,
+		},
+	})
+
+	if err := app.Run(); err != nil {
+		log.Fatal(err)
 	}
 }