fix(compile): resolve net48 compatibility errors CS0117, CS1061, CS1503

[ORagoter]

• PatchHearthstone.cs: replaced TryAdd with indexer assignment
• WebServer.cs: replaced ReadAllBytesAsync with synchronous ReadAllBytes
• WebPage.cs: fixed Split method syntax for net48

概述 Overview

实现/解决/优化的内容（implemented/solved/optimized/fixed:）:
Fixed BepInEx 5 branch compilation break on net48 target framework when using modern .NET SDK tools (errors CS0117, CS1061, CS1503).

检查 Check

功能 Function

• 若有新增，已编写完善的配置文件字段说明，并确保enUS.json已完善 (If there are new additions, complete descriptions of the configuration file fields have been written, and enUS.json has been completed.)
• 若有必要，已编写面向用户的新功能说明 (If necessary, user-facing descriptions of new features have been written)
• 已测试新功能或更改 (New features or changes tested)