Introduce theming contract and window configuration

[mdwigley]

Description

This PR introduces a minimal theming contract for the PlugHub host, adds structured window configuration via AppConfig and AppEnv, and performs a cleanup pass on resources and formatting.

• Theming Contract (131)

  • Added Style/Generic.axaml defining PlugHub.* resource keys for brushes, typography, and layout metrics.
  • Introduced a theme configuration pipeline in App.axaml.cs to normalize and apply IStyle additions.
  • Adjusted bootstrapper initialization order so the contract is established before pages are created.
  • Removed redundant Avalonia.Themes.Fluent dependency.

• Window Configuration (132)

  • Extended AppConfig (global baseline) and AppEnv (runtime overrides) with window configuration values.
  • Bound MainWindow properties to these configuration sources.
  • Ensured MainView’s ViewModel drives window configuration.
  • Clarified contract: overriding MainView/MainViewModel implies responsibility for maintaining the MainWindow configuration contract.

• Cleanup (133)

  • Removed unused geometry stream icons from Icons.axaml.
  • Standardized all .axaml files to use tab indentation.

Related Issue

• Resolves Minimal theming contract for PlugHub host #131
• Resolves Window Configuration via Host Config #132
• Resolves Codebase Cleanup and Resource Consolidation #133

Motivation and Context

• Establishes a stable theming abstraction decoupled from FluentAvalonia while still providing a polished baseline.
• Provides a structured, user‑first mechanism for configuring window properties through config/env.
• Clarifies architectural boundaries: pages are free, but MainView ownership carries window responsibilities.
• Improves maintainability and consistency by cleaning up unused resources and enforcing formatting standards.

How Has This Been Tested?

• Verified theming contract keys resolve correctly when bound from host pages.
• Tested theme override pipeline by injecting alternate styles.
• Confirmed MainWindow properties update correctly from both AppConfig and AppEnv, with user overrides taking precedence.
• Validated fallback logic when user config values are missing.
• Manual smoke testing across supported platforms to ensure no regressions in host startup, navigation, or plugin page rendering.

Screenshots (if appropriate):

N/A

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.