Fix Browser theme MDI chrome and split drops

[wieslawsoltes]

Summary

• restore native main-window chrome when the BrowserTabTheme sample switches to MDI layout mode
• expose the full document docking selector so side drops create horizontal or vertical document docks
• document both behaviors and add a Browser theme resource regression test

Validation

• dotnet test tests/Dock.Avalonia.Themes.UnitTests/Dock.Avalonia.Themes.UnitTests.csproj -c Release --no-restore (64 passed)
• dotnet build samples/BrowserTabTheme/BrowserTabTheme.csproj -c Release --no-restore
• dotnet build Dock.slnx -c Release --no-restore

Closes #1056

[chatgpt-codex-connector]

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Reviewed commit: 1f87eda595

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[chatgpt-codex-connector]

💡 Codex Review

Dock/samples/BrowserTabTheme/App.axaml.cs

Line 77 in 93f049d

desktop.Exit += (_, _) => DisposeMainWindowViewModel();

Move exit cleanup out of code-behind

In the desktop lifetime path, this lambda introduces a code-behind event handler solely to manage ViewModel disposal, contrary to the repository's explicit ReactiveUI rule forbidding event handlers in code-behind. Route the shutdown cleanup through a ReactiveUI lifecycle abstraction, behavior, or service instead of subscribing directly in App.axaml.cs.

AGENTS.md reference: AGENTS.md:L67-L70

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• Mark a draft as ready
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