feat: add test excalidraw page with deep linking support

- Added new test excalidraw page component and route with deep linking via ?tests=excalidraw
- Enhanced drawings editor to preserve frontmatter and normalize metadata when saving .excalidraw.md files
- Improved excalidraw file handling with better error handling and sanitization
- Updated sidebar and navigation to include test excalidraw option
- Fixed collaborators map serialization issue in excalidraw scene data

src/app.component.simple.html

@@ -30,6 +30,7 @@
     (markdownPlaygroundSelected)="setView('markdown-playground')"
     (parametersOpened)="setView('parameters')"
     (testsPanelRequested)="setView('tests-panel')"
+    (testsExcalidrawRequested)="setView('tests-excalidraw')"
     (helpPageRequested)="openHelpPage()"
     (noteCreated)="onNoteCreated($event)"
     (noteCreatedAndSelected)="onNoteCreatedAndSelected($event)"
@@ -541,6 +542,8 @@
           <app-parameters></app-parameters>
         } @else if (activeView() === 'tests-panel') {
           <app-tests-panel></app-tests-panel>
+        } @else if (activeView() === 'tests-excalidraw') {
+          <app-test-excalidraw-page></app-test-excalidraw-page>
         } @else {
           @if (activeView() === 'drawings') {
             @if (currentDrawingPath()) {