refactor: use theme constants for environment label styling

[liias]

#384 extracted the focus fix from #341 and this PR here rebases on top of new main branch.

• Replace hardcoded font size and color with MainWindowTheme::ENV_PROFILE_LABEL_FONT and ENV_PROFILE_LABEL_COLOR.
• Aligns label appearance with the app's theming system for consistent UI.
• Simplifies future theme customizations and reduces duplicated style values.

feat: Add customizable hotkey background and text colors, including hover states, to appearance settings and apply them to hotkey labels.

Remove redundant SET_FOCUSED_INDEX command submission from else block.

feat: Add UI options for custom font sizes and families, and implement browser focus state.

feat: add focus‑specific env customization and hover color settings

• Introduce FocusWidget::with_env_on_focus to apply a custom Env when the widget gains focus, affecting event, lifecycle, update, layout, and paint handling.
• Extend MainWindowTheme with new keys ENV_HOVER_TEXT_COLOR and ENV_HOVER_SECONDARY_TEXT_COLOR and propagate them through theme initialization and environment setup.
• Add corresponding fields (hover_text_color, hover_secondary_text_color) to the theme struct and default values in CustomTheme.
• Update UI settings view to expose color inputs for hover text, secondary text, and hover secondary text, enabling user customization.
• Adjust theme parsing to read the new color values from CustomTheme and apply them to the main window theme.

feat: add customizable hover background color support

• Add hover_background to CustomTheme with default RGBA value
• Extend MainWindowTheme with hover_background_color and ENV key
• Parse and apply the custom hover color when building the theme
• Introduce a color input in the Appearance settings to let users edit the hover background
• Update main window widget rendering to use the new env color for hover/selected states
• Minor cleanup of imports and minor UI adjustments related to the new feature

feat: add custom appearance settings with theme customization

• Introduce an Appearance tab in the settings window allowing users to select theme mode and customize colors.
• Replace the old ConfiguredTheme with a new ThemeMode enum that includes a Custom option.
• Add a CustomTheme struct to store hex color values and provide default theme colors.
• Extend UIVisualSettings and UIState to hold theme_mode and custom_theme data.
• Update theme initialization to apply custom colors when ThemeMode::Custom is selected, including a color parser.
• Modify settings UI to include radio buttons for theme selection and text boxes for custom color inputs, with automatic save handling.