feat: persist last user + diagnostics + logging + icon + defensive whoami

Backend:
- last_user.go: writes/reads <UserConfigDir>/matrix_client_pc/last_user.txt.
  Login persists; Logout clears.
- GetLastUserID bind replaces fragile localStorage in App.tsx.
- GetDiagnostics bind: live snapshot (started, client_ready, crypto_init,
  sync_active, rooms_count, encrypted_rooms, dms_count, last_error).
- applog.go: slog to stderr + <UserConfigDir>/matrix_client_pc/app.log.
  GetLogTail + GetLogPath binds.
- matrix_service.go logs throughout Login/Start. After MatrixClientInit,
  if client.DeviceID empty -> retry whoami + persist back (defensive).
- main.go inits logger before wails.Run, OnShutdown logs close.

Frontend:
- App.tsx awaits GetLastUserID() instead of localStorage.
- HomeScreen.tsx Health modal (green stethoscope button) with HealthRow
  status dots — comprobar chats.
- Auto-relogin on token-rejected error in Start().

Icon:
- appicon.ico (Phosphor chat-circle + #7c3aed) generated via generate_app_icon.
- build/windows/icon.ico replaced (Wails embeds via windres).
- build/appicon.png regenerated from ico (256x256).

Refs: issues 0147 + 0148 + 0150 (partial). Fixes M_UNKNOWN_TOKEN auto-recovery.

main.go

@@ -14,9 +14,16 @@ import (
 var assets embed.FS
 
 func main() {
+	logger, err := InitLogger()
+	if err != nil {
+		log.Fatalln("logger init:", err)
+	}
+	defer logger.Close()
+	logger.Info("starting matrix_client_pc", "version", "0.1.0")
+
 	ms := NewMatrixService()
 
-	err := wails.Run(&options.App{
+	err = wails.Run(&options.App{
 		Title:  "matrix_client_pc",
 		Width:  1280,
 		Height: 800,
@@ -26,12 +33,17 @@ func main() {
 		BackgroundColour: &options.RGBA{R: 26, G: 27, B: 30, A: 1},
 		OnStartup: func(ctx context.Context) {
 			ms.SetContext(ctx)
+			logger.Info("wails ctx ready")
+		},
+		OnShutdown: func(ctx context.Context) {
+			logger.Info("shutdown")
 		},
 		Bind: []interface{}{
 			ms,
 		},
 	})
 	if err != nil {
+		logger.Error("wails error", "err", err)
 		log.Fatalln("Wails error:", err)
 	}
 }