feat(excalidraw): support complet des fichiers .excalidraw (#78)

- Backend: ajout .excalidraw dans SUPPORTED_EXTENSIONS + squelette JSON creation
- Backend: détection is_excalidraw dans api_file() + FileContentResponse
- Frontend: icône pen-tool dans EXT_ICONS
- Frontend: ajout dans la modale de création (ui.js)
- Frontend: excalidraw-editor.html (iframe autonome, React+Excalidraw via esm.sh)
- Frontend: excalidraw-viewer.js (postMessage, auto-save 2s, thème)
- Frontend: dispatch dans viewer.js (renderFile)
- Tests: 6 tests (détection, création squelette, fallback, tree, roundtrip)
- 359/359 tests passent

backend/indexer.py

@@ -63,6 +63,7 @@ SUPPORTED_EXTENSIONS = {
     ".java", ".c", ".cpp", ".h", ".hpp", ".cs", ".go", ".rs", ".rb",
     ".php", ".sql", ".r", ".m", ".swift", ".kt",
     ".dockerfile", ".makefile", ".cmake",
+    ".excalidraw",
 }