Fix memory leak / assertion error when switching to schemas with custom icons

[cqjjjzr]

在 Debug 模式下如果切换到有自定义图标的方案时会引起 Assertion Error 而崩溃，因为在往一个已经有数据的 CIcon 上 LoadIconW。

这里把内建和自定义图标的 CIcon 分开管理了，并确保在已经有 HICON != nullptr 的 CIcon 上加载时会先销毁旧的。

[Copilot]

Pull request overview

This PR fixes a crash (debug assertion) and potential icon-handle leakage when switching to schemas that use custom status icons, by separating built-in and custom icon storage and ensuring old custom icons are destroyed before reloading.

Changes:

• Introduces dedicated CIcon members to hold custom schema icons separately from built-in resource icons.
• Replaces the previous “load into the same CIcon” logic with a helper that destroys any existing handle before attaching a newly loaded icon.
• Adds a simple “custom if successfully loaded, otherwise built-in” selection path when painting the status icon.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
WeaselUI/WeaselPanel.h	Adds CIcon members for custom icons to keep them separate from built-in icons.
WeaselUI/WeaselPanel.cpp	Implements custom icon loading with explicit destroy/attach and uses a fallback to built-in icons during paint.

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