feat: improve chunk viewer with fixed navigation and markdown rendering

This commit implements three UI improvements for the chunk viewer:

1. Fixed modal footer with navigation controls
   - Moved PDF navigation buttons to a fixed footer
   - Footer remains visible while scrolling content
   - Three-section layout: fixed header, scrollable body, fixed footer

2. Removed duplicate navigation controls
   - Removed previous/next buttons from PDFViewer component
   - Controls now only in App.vue modal footer
   - Cleaned up unused imports and CSS

3. Markdown rendering for chunk content
   - Created MarkdownViewer component using markdown-it
   - Renders markdown content aligned with Nextcloud design system
   - Removed problematic markdown-it-task-checkbox dependency
   - Combines before/chunk/after context with visual separators

4. Cleaned up search results display
   - Removed excerpt snippets from results list
   - Kept only chunk/page metadata for cleaner UI

The modal structure now has:
- Fixed header (title + close button)
- Scrollable body (PDF canvas or markdown content)
- Fixed footer (page navigation - always visible)

Fixes markdown rendering "require is not defined" error by using
only markdown-it without CommonJS plugins.

third_party/astroglobe/package.json

@@ -22,6 +22,7 @@
     "@nextcloud/l10n": "^3.1.0",
     "@nextcloud/router": "^3.0.1",
     "@nextcloud/vue": "^8.29.2",
+    "markdown-it": "^14.1.0",
     "plotly.js-dist-min": "^2.35.3",
     "pdfjs-dist": "^4.0.379",
     "vue": "^2.7.16",