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.

app.go

@@ -1,58 +1,49 @@
 package main
 
 import (
-	"context"
 	_ "embed"
-	"log"
 	"strings"
 
-	"github.com/wailsapp/wails/v2/pkg/options"
-	"github.com/wailsapp/wails/v2/pkg/runtime"
-
 	"github.com/tidwall/gjson"
+	"github.com/wailsapp/wails/v3/pkg/application"
 )
 
 //go:embed wails.json
 var wailsJSON string
 
-var wailsContext *context.Context
-
 // App struct
 type App struct {
-	ctx context.Context
+	app *application.App
 }
 
 // NewApp creates a new App application struct
-func NewApp() *App {
-	return &App{}
+func NewApp(app *application.App) *App {
+	return &App{app: app}
 }
 
-// startup is called when the app starts. The context is saved
-// so we can call the runtime methods
-func (a *App) startup(ctx context.Context) {
-	version := gjson.Get(wailsJSON, "info.productVersion")
-	wailsContext = &ctx
-	runtime.WindowSetTitle(ctx, "AniTrack "+version.String())
-	//runtime.WindowMaximise(ctx)
+// appVersion reads the product version from wails.json.
+func appVersion() string {
+	return gjson.Get(wailsJSON, "info.productVersion").String()
 }
 
-func (a *App) onSecondInstanceLaunch(secondInstanceData options.SecondInstanceData) {
-	var secondInstanceArgs = secondInstanceData.Args
+// appTitle is the window title: "AniTrack <version>".
+func appTitle() string {
+	return "AniTrack " + appVersion()
+}
 
-	println("user opened second instance", strings.Join(secondInstanceData.Args, ","))
-	println("user opened second from", secondInstanceData.WorkingDirectory)
-	runtime.WindowUnminimise(*wailsContext)
-	runtime.Show(*wailsContext)
-	go runtime.EventsEmit(*wailsContext, "launchArgs", secondInstanceArgs)
+func (a *App) onSecondInstanceLaunch(data application.SecondInstanceData) {
+	println("user opened second instance", strings.Join(data.Args, ","))
+	println("user opened second from", data.WorkingDir)
+	if w := a.app.Window.Current(); w != nil {
+		w.Restore()
+		w.Focus()
+	}
+	go a.app.Event.Emit("launchArgs", data.Args)
 }
 
 func (a *App) ShowVersion() {
-	version := gjson.Get(wailsJSON, "info.productVersion")
-	_, err := runtime.MessageDialog(*wailsContext, runtime.MessageDialogOptions{
-		Title:   "Version",
-		Message: "AniTrack Version: " + version.String(),
-	})
-	if err != nil {
-		log.Println(err)
-	}
+	a.app.Dialog.Info().
+		SetTitle("Version").
+		SetMessage("AniTrack Version: " + appVersion()).
+		Show()
 }