Refresh WPF UI and add safer spoofing workflow

[henriksb]

Motivation

• Modernize the old WPF UI for a clearer, more discoverable workflow (select source, configure visible/real extension, preview, generate).
• Make the tool safer and easier to use by preventing accidental renames, giving clearer status/feedback, and supporting copy mode.
• Improve icon handling and performance by scanning only .ico files and caching icon paths for reliable previews.

Description

• Reworked the main window layout in ExtensionSpoof/ExtensionSpoof/MainWindow.xaml to a two-panel design with status text, drag-and-drop support, output folder picker, preview area, and a CopyMode option.
• Rewrote ExtensionSpoof/ExtensionSpoof/MainWindow.xaml.vb to add input validation, safer generation flow (default copy instead of rename), duplicate-output checks, drag-and-drop and output-folder actions, and status/feedback via SetStatus.
• Added icon scanning improvements: AddSpoofExtensions now accepts an icon folder path, enumerates only *.ico files, deduplicates/sorts names, and caches full icon paths in IconMap for fast live-preview updates.
• Small refactors and defaults: preselects sensible extension/output folder values, sets a default name, and surfaces clearer error messages for partial failures (e.g., icon injection).

Testing

• Attempted to build with msbuild ExtensionSpoof.sln but the msbuild tool was not available in the container, so the build could not run.
• Attempted to build with dotnet build ExtensionSpoof.sln -c Release but the .NET SDK was not available in the container, so the build could not run.

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Codex Task