feat: implement chat history, theme customization, and password management

- Added chat history persistence for active rooms with auto-cleanup on stream end.
- Overhauled Settings page with user profile, theme color picker, and password change.
- Added backend API for user password updates.
- Integrated flutter_launcher_icons and updated app icon to 'H' logo.
- Fixed 'Duplicate keys' bug in danmaku by using UniqueKey and filtering historical messages.
- Updated version to 1.0.0-beta3.5 and author info.

backend/internal/api/router.go

@@ -27,6 +27,7 @@ func SetupRouter() *gin.Engine {
 	authGroup.Use(AuthMiddleware())
 	{
 		authGroup.GET("/room/my", GetMyRoom)
+		authGroup.POST("/user/change-password", ChangePassword)
 	}
 
 	return r