feat: ✨ add theme editor

[amd989]

Theme Editor for LCD4Linux AX206

Adds a visual theme editor, a standalone Avalonia desktop application for creating and editing LCD4Linux theme configuration files without hand-editing XML.

Features

• Visual canvas: widgets are rendered on a 1:1 pixel representation of the display. Drag to reposition, click to select.
• Property panel: context-sensitive editor for each widget's properties. Supports text, numeric, boolean, color, class, and alignment field types.
• Undo/redo: full history for move and property edit operations.
• Load/Save: reads and writes the native LCD4Linux config format via ConfigParser / ConfigSerializer.
• Zoom: adjustable canvas zoom (0.5x-3x) with scroll wheel support.
• Rubber-band selection: drag to select multiple widgets at once.
• Dark UI: Fluent dark theme with teal accent, styled property panel, and hover feedback.

Technical notes

• Built with Avalonia 11 (Fluent theme, ColorPicker extension)
• MVVM architecture: MainWindowViewModel, WidgetViewModel, PropertyViewModel
• CI pipeline extended to build and publish the editor binary alongside the existing firmware artifacts