Fix/5009 titlebar search text styling

[Gabriel Dufresne (GabrielDuf)]

This pull request improves the visual consistency and accessibility of placeholder text and title bar elements in the UI, aligning them more closely with WinUI standards. The main changes include introducing theme-specific placeholder colors, making placeholder opacity consistent, and updating title bar and sidebar styling for better focus indication and readability.

Placeholder Text Styling Improvements:

• Added SearchBoxPlaceholderForeground and MegaQueryPlaceholderForeground resources in Styles.Common.axaml for both light and dark themes, ensuring appropriate placeholder color contrast and weight matching WinUI conventions.
• Applied the new placeholder foreground resources to the global search box and the main "mega query" search box, ensuring consistent placeholder appearance across the app.
• Set TextControlPlaceholderOpacity to 1 in relevant TextBox resources, so placeholder text displays at full intended brightness, matching WinUI behavior.

Title Bar and Window Focus Enhancements:

• Removed hard-coded opacity from title bar elements and sidebar window buttons, and implemented SetupTitleBarFocusOpacity() in MainWindow.axaml.cs to dynamically adjust opacity based on window focus, giving a clear visual cue when the window is active or inactive.
  Sidebar and Search Box Font Weight Adjustments:

• Changed several sidebar and search box text elements from SemiBold to Normal font weight for improved consistency and readability.