docs: mention native context menu theme support

[HavenDV]

Summary

• mention native light/dark theme support in the README feature list
• document ContextMenuThemeMode defaults and overrides for WinUI context menus
• add a compact XAML example for forcing a native dark tray context menu

Validation

• dotnet build H.NotifyIcon.PR.slnx --configuration Release --nologo
• git diff --check -- readme.md (no whitespace errors; Git reported the existing CRLF checkout warning)

[claude]

Review: docs: mention native context menu theme support

This is a clean, well-scoped, docs-only PR (readme.md, +11/-2). I cross-checked every technical claim against the source and they all hold up:

• ContextMenuThemeMode default is indeed PopupMenuThemeMode.System (TaskbarIcon.ContextMenu.WinRT.cs:7), and the enum docs confirm System follows the Windows app theme while Default uses "the Windows default native menu app mode" (PopupMenuThemeMode.cs:6-26) — matches the new wording exactly.
• The claim that SecondWindow mode also respects ContextMenuThemeMode for its WinUI flyout theme is accurate — see ApplySecondWindowContextMenuTheme in TaskbarIcon.ContextMenu.WinUI.SecondWindow.cs:298-312, which maps System → ActualTheme, Light/Dark → ElementTheme.Light/Dark.
• H.NotifyIcon.Core.PopupMenu.ThemeMode exists and defaults to PopupMenuThemeMode.System (PopupMenu.cs:27), consistent with "Direct H.NotifyIcon.Core.PopupMenu users can set the same behavior with the ThemeMode property."
• The new XAML snippet's formatting (closing /> on its own line, no xmlns shown) matches the conventions already used elsewhere in the readme.

Nothing to flag for bugs/security/performance since this changes no code — N/A for those review dimensions here. Two minor, non-blocking suggestions:

1. The doc only mentions theme support for PopupMenu and SecondWindow modes. Worth double-checking (or a follow-up note) whether ActiveWindow mode intentionally doesn't apply ContextMenuThemeMode — I didn't find any reference to it in TaskbarIcon.ContextMenu.WinRT.cs for that mode, so readers might wonder why it's omitted.
2. ContextMenuMode="PopupMenu" in the new example is the existing default, so it's technically redundant — but keeping it explicit is reasonable here since the surrounding prose is specifically about PopupMenu mode, so I'd leave it as-is.

Good validation steps in the PR description (build + whitespace check). Approving from a content-accuracy standpoint.