feat(image): Bessere Bildanzeige & Interaktive Vorschau (ImagePreviewDialog)

[lukisch]

Summary of Changes

1. Interactive Image Preview Dialog (ImagePreviewDialog):

   • High-resolution preview window with Zoom (+/- / 1:1 / Fit to Window), Clipboard Image Copy, and metadata resolution.
   • Triggers automatically when clicking on image anchors in the markdown preview pane.

2. Structured Figure & Caption HTML Rendering (_render_figures_and_captions):

   • Wraps markdown images in standard HTML <figure> and <figcaption> elements using image title or alt text.
   • Encapsulates images in clickable <a href="..."> links for instant click-to-zoom preview.

3. Enhanced Theme Styling (CSS):

   • Modern HSL/RGB responsive image styles in dark and bright themes: rounded corners (border-radius: 8px), elevation shadows, subtle border frames, and centered italic captions.

4. Image Insertion UX (_insert_image):

   • Native file picker (QFileDialog) with automatic relative path resolution relative to the open document.
   • Pre-populates default alt text based on file stem. Fallback URL entry when canceled.

5. 6-Language i18n Support & Test Coverage:

   • Translations added to locales/translations.json for de, en, es, zh, ja, ru.
   • 91/91 unit tests passing (Pytest) and 100% PASS on python main.py --self-test.