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

frontend/js/viewer.js

@@ -7,6 +7,7 @@ import { syncActiveFileTreeItem, searchByTag, TagFilterService, refreshSidebarTr
 import { AutocompleteDropdown, performAdvancedSearch } from './search.js';
 import { initDashboardTabs } from './sync.js';
 import { renderMermaidBlocks } from './mermaid-viewer.js';
+import { renderExcalidraw } from './excalidraw-viewer.js';
 import { DashboardStatsWidget, DashboardRecentWidget, DashboardBookmarkWidget, DashboardSharedWidget, DashboardConflictsWidget } from './dashboard.js';
 import { openShareDialog } from './config.js';
 import { cacheViewedFile, getCachedFile } from './offline.js';
@@ -448,6 +449,12 @@ export function renderFile(data) {
     return;
   }
 
+  // Handle Excalidraw — render in iframe editor
+  if (data.is_excalidraw) {
+    renderExcalidraw(area, data, data.vault, data.path);
+    return;
+  }
+
   // Handle images
   if (data.is_image) {
     const imgUrl = `/api/file/${encodeURIComponent(data.vault)}/raw?path=${encodeURIComponent(data.path)}`;