feat(astrolabe): add admin search settings and enhanced UI

Add comprehensive admin controls for the unified search provider and enhance the frontend UI with filtering and visualization improvements.

**Admin Settings:**
- Configure default search algorithm (hybrid, semantic, bm25)
- Set fusion method for hybrid search (rrf, dbsf)
- Adjust minimum score threshold (0-100%)
- Set result limit (1-100 results)

**Frontend Enhancements:**
- Add score-based result filtering with slider control
- Add expandable excerpts for search results
- Improve result visualization and formatting
- Add algorithm badge to show search method used

**API Changes:**
- Add `/api/admin/search-settings` POST endpoint
- Add `searchForUnifiedSearch()` method to McpServerClient
- Load and apply admin settings in SemanticSearchProvider

**Technical Details:**
- Settings stored in app config table
- Defaults: hybrid algorithm, rrf fusion, 0% threshold, 20 results
- SemanticSearchProvider respects admin-configured limits

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

third_party/astroglobe/lib/Settings/AdminSection.php

@@ -47,6 +47,6 @@ class AdminSection implements IIconSection {
 	 * @return string Section icon (SVG or image URL)
 	 */
 	public function getIcon(): string {
-		return $this->urlGenerator->imagePath('astroglobe', 'app.svg');
+		return $this->urlGenerator->imagePath('astroglobe', 'app-dark.svg');
 	}
 }