feat: initialize elements and files in INITIAL_APP_DATA and normalize canvas data on pad initialization

- Added `elements` and `files` properties to `INITIAL_APP_DATA` for better state management.
- Updated the `usePad` hook to normalize canvas data before updating the scene, enhancing data consistency.

Author: atyrode

src/frontend/src/constants.ts

@@ -6,6 +6,8 @@ export const INITIAL_APP_DATA = {
         gridSize: 20,
         gridStep: 5,
     },
+    elements: [],
+    files: [],
 };
 
 // UI elements

src/frontend/src/hooks/usePadData.ts

@@ -43,7 +43,8 @@ export const usePad = (padId: string | null, excalidrawAPI: ExcalidrawImperative
     useEffect(() => {
         if (isTemporaryPad && excalidrawAPI) {
             console.debug(`[pad.ws] Initializing new temporary pad ${padId}`);
-            excalidrawAPI.updateScene(INITIAL_APP_DATA);
+            const normalizedData = normalizeCanvasData(INITIAL_APP_DATA);
+            excalidrawAPI.updateScene(normalizedData);
             return; 
         }