优化深色模式支持

[Glavo]

1. 修复 macOS 上主题模式设置无法跟随系统设置的问题
2. 在低版本 JavaFX 上支持跟随系统设置（目前仅支持 Windows/macOS，且仅在启动时检测一次，不会在运行中持续检测主题模式变化）

[Copilot]

Pull request overview

This PR optimizes dark mode support by fixing theme mode settings on macOS to follow system settings and adding fallback support for system theme detection on lower JavaFX versions (currently Windows/macOS only, detected once at startup).

Changes:

• Always show "auto", "light", and "dark" theme options in the UI regardless of JavaFX version
• Add fallback system theme detection for older JavaFX versions using native APIs (Windows registry and macOS defaults command)
• Set apple.awt.application.appearance to "system" on macOS to enable system theme following

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
HMCL/src/main/java/org/jackhuang/hmcl/ui/main/PersonalizationPage.java	Removed conditional logic that hid the "auto" theme option on older JavaFX versions, now always shows all three options
HMCL/src/main/java/org/jackhuang/hmcl/theme/Themes.java	Added getDefaultBrightness() method to detect system theme via Windows registry or macOS defaults command when JavaFX native support is unavailable
HMCL/src/main/java/org/jackhuang/hmcl/EntryPoint.java	Set apple.awt.application.appearance system property to "system" on macOS for proper AWT/Swing theme following

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[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.

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