A11y: Fix tab order, focus, dark mode, and Name property issues

[haoliuu]

First batch of fixes for accessibility bugs reported against AI Dev Gallery.

Fixes

• Home page tab order: Preview link now receives focus before the carousel tiles when tabbing from the left nav.
• Custom Parameters sample: Focus stays on Generate after invocation (moves to Stop while generating, returns to Generate when done).
• Dark mode visibility: OpenAI logo and title bar Min/Max/Close glyphs are now visible in dark mode and follow runtime theme changes.
• UIA Name property on code panel: CodeTextBlock and LineNumbersTextBlock now expose concise AutomationProperties.Name values instead of the full source / digit sequence (was exceeding 512 characters).
• 58929691/58932882/58944323/58946033

Validation

Verified manually with keyboard navigation, dark/light theme switching,
and Accessibility Insights for Windows.

[Copilot]

Pull request overview

This PR delivers a first batch of accessibility and theming fixes in AI Dev Gallery, improving keyboard navigation/focus behavior, UI Automation (UIA) naming, and dark-mode (theme-aware) icon/title bar rendering.

Changes:

• Adjusted home header keyboard navigation order (preview hyperlink before carousel tiles) via TabIndex.
• Improved focus behavior in the Custom Parameters (CustomSystemPrompt) sample by moving focus to Stop while generating and restoring focus to Generate when appropriate.
• Made dark/light theme changes propagate to model picker branding assets and window caption button colors, and set concise UIA names for the code panel.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
AIDevGallery/Utils/AppUtils.cs	Makes theme-asset suffix selection use the window’s resolved ActualTheme when available.
AIDevGallery/Samples/Open Source Models/Language Models/CustomSystemPrompt.xaml.cs	Improves keyboard focus behavior during/after generation and on cancel.
AIDevGallery/MainWindow.xaml.cs	Updates caption button colors on runtime theme changes and refreshes them when resources update.
AIDevGallery/Controls/SampleContainer.xaml.cs	Sets a concise UIA Name for the code panel after rendering code.
AIDevGallery/Controls/SampleContainer.xaml	Adds concise AutomationProperties.Name values for line numbers and code display elements.
AIDevGallery/Controls/ModelPicker/ModelPickerViews/OpenAIPickerView.xaml.cs	Updates the OpenAI logo to switch based on runtime theme changes.
AIDevGallery/Controls/ModelPicker/ModelPickerViews/OpenAIPickerView.xaml	Adds x:Name so the OpenAI logo image can be updated programmatically.
AIDevGallery/Controls/HomePage/Header/HeaderCarousel.xaml	Adjusts tab order so the preview link is reachable before the carousel tiles.