Fix dark mode bug on second instance dialog and SVG preview default theme

[BerryUIKI]

PR Checklist

• Functionality has been tested, no obvious bugs
• Code style follows project conventions
• Documentation/comments updated (if applicable)

Brief Description of Changes

1. Fix duplicate instance reminder dialog theme:
   • In App.xaml.cs, EnsureFirstInstance previously called MessageBox.Show before ThemeManager.Apply(...) and MessageBoxPatcher.Initialize() were executed. This caused the duplicate instance notification dialog to stay in light mode even when Windows was in dark mode.
   • Added an idempotency check _isInitialized in MessageBoxPatcher.Initialize() and ensured ThemeManager.Apply and MessageBoxPatcher.Initialize are initialized before showing the message box in EnsureFirstInstance.
2. Follow system theme for SVG and image previews:
   • In QuickLook.Plugin.ImageViewer, context.Theme hardcoded 1 (Themes.Dark) as fallback when reading LastTheme.
   • Updated Plugin.cs and Webview/WebHandler.cs to default to OSThemeHelper.AppsUseDarkTheme() ? Themes.Dark : Themes.Light.
   • Updated OPTIONS.md documentation for to reflect the default value

Summary by Sourcery

Fix theme initialization for duplicate-instance dialogs and default image previews to the operating system theme.

Bug Fixes:

• Ensure duplicate-instance notification dialogs use the system light or dark theme.
• Make SVG and image previews follow the system theme when no explicit preview theme is configured.

Enhancements:

• Make message-box patch initialization safe to invoke multiple times.

Documentation:

• Update ImageViewer options documentation to describe system-theme behavior for the default preview theme.

[sourcery-ai]

Reviewer's Guide

The PR fixes dark-mode handling for duplicate-instance dialogs by applying the theme and MessageBox patches before the dialog is shown, with idempotent patch initialization. It also changes ImageViewer SVG/image previews to follow the system theme when LastTheme is unset or None, and aligns the configuration documentation with that behavior.

Sequence diagram for themed duplicate-instance dialog

sequenceDiagram
    participant App as QuickLookApp
    participant ThemeManager
    participant OSThemeHelper
    participant MessageBoxPatcher
    participant MessageBox

    App->>App: EnsureFirstInstance(args)
    App->>ThemeManager: Apply(OSThemeHelper.AppsUseDarkTheme())
    App->>MessageBoxPatcher: Initialize()
    alt patcher already initialized
        MessageBoxPatcher-->>App: return
    else first initialization
        MessageBoxPatcher-->>App: apply Harmony patches
    end
    App->>MessageBox: Show(duplicate instance notification)
    MessageBox-->>App: OK

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Flow diagram for system-based ImageViewer preview theme

flowchart TD
    A[Prepare or TryPrepare preview] --> B[Read LastTheme]
    B --> C{LastTheme is None or unset?}
    C -->|Yes| D["OSThemeHelper.AppsUseDarkTheme()"]
    C -->|No| E[Use configured LastTheme]
    D --> F[Set context.Theme to Dark or Light]
    E --> F

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File-Level Changes

Change	Details	Files
Initializes application theming and MessageBox patching before any duplicate-instance notification is displayed, while making patch setup idempotent.	Centralizes theme and patcher initialization in a reusable startup helper. Invokes the helper from the second-instance path before showing the dialog. Guards Harmony patch registration against repeated initialization.	QuickLook/App.xaml.cs QuickLook/Helpers/MessageBoxPatcher.cs
Makes image and SVG preview themes follow the operating-system theme when no explicit preview theme is configured.	Uses the current system dark/light preference as the LastTheme fallback. Resolves the None setting to the current system theme in both preview preparation paths. Updates LastTheme documentation to describe the system-following default.	QuickLook.Plugin/QuickLook.Plugin.ImageViewer/Plugin.cs QuickLook.Plugin/QuickLook.Plugin.ImageViewer/Webview/WebHandler.cs OPTIONS.md

Possibly linked issues

• #SVG preview window is always dark: PR changes ImageViewer defaults and None handling to follow system light/dark theme, directly fixing dark SVG previews.
• #Fix Dark Mode Bug: The PR initializes theming before the duplicate-instance message box, directly fixing the reported light-mode dialog bug.

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[sourcery-ai]

Hey - I've reviewed your changes and they look great!

Sourcery assessment

Approved.

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