Standardized Theming Structure for UI Assets

[mdwigley]

Description

This change restructures theming so that resources are organized per theme family rather than in a single catch‑all file.

• Theming changes

  • Removed legacy Generic.axaml and Icons.axaml catch‑all files.
  • Introduced Themes/FluentAvalonia/Theme.axaml for brushes and geometry resources.
  • Added Themes/FluentAvalonia/Style.axaml (currently minimal/placeholder) to establish the per‑family structure.
  • Updated App.axaml.cs to merge the new theme dictionaries.
  • Refactored MainView.axaml, SettingsPluginsView.axaml, and SettingsView.axaml to consume PlugHub.* resource keys instead of hard‑coded values.

• Plugin updates

  • Same as the what was done for the host.
  • Updated plugin views (e.g. MockSettingsView) to use PlugHub.* brushes for backgrounds, borders, and text, ensuring theme‑family consistency.
  • Fixed a sorting issue in descriptors list that was caused by incorrect index values rather than logic errors.

Related Issue

• Resolves Standardize Theming Structure for UI Assets #138

Motivation and Context

• Moves away from a single Generic.axaml dumping ground and aligns with Avalonia’s theming conventions.
• Establishes a clear, maintainable structure for theme families (Theme.axaml + Style.axaml).
• Ensures plugins also respect the centralized theming model, so they look consistent across themes.
• Corrects plugin resource ordering so that resources load deterministically.

How Has This Been Tested?

• Verified that the application builds and runs with the new theme structure.
• Confirmed that views (MainView, SettingsPluginsView, SettingsView) render correctly with the new brushes.
• Tested plugin views to ensure they pick up the correct theme brushes.
• Validated that plugin resource dictionaries load without duplication and in the correct order after the sorting fix.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Asset change (adds or updates icons, templates, or other assets)
• Documentation change (adds or updates documentation)
• Plugin change (adds or updates a plugin)

Checklist:

• I have read the CONTRIBUTING document.
• My change requires a change to the core logic.
  • I have linked the project issue above.
• My change requires a change to the assets.
  • I have linked the asset issue above.
• My change requires a change to the documentation.
  • I have linked the documentation issue above.
• My change requires a change to a plugin.
  • I have linked the plugin issue above.