Fix fabric.js v6 import error causing blank page

- Replace deprecated 'import { fabric } from fabric' with named imports
- Editor.vue: import Canvas, Rect, FabricText
- symmetry.js: import FabricImage, Rect, Group
- Clear Vite cache to rebuild dependencies

frontend/node_modules/.vite/deps/_metadata.json

@@ -1,49 +1,43 @@
 {
-  "hash": "76c4bd08",
-  "configHash": "3c200fce",
+  "hash": "8c4b68ba",
+  "configHash": "34f6f529",
   "lockfileHash": "99e89a35",
-  "browserHash": "824a39cf",
+  "browserHash": "6a02d3bf",
   "optimized": {
-    "@element-plus/icons-vue": {
-      "src": "../../@element-plus/icons-vue/dist/index.js",
-      "file": "@element-plus_icons-vue.js",
-      "fileHash": "18428861",
-      "needsInterop": false
-    },
     "axios": {
       "src": "../../axios/index.js",
       "file": "axios.js",
-      "fileHash": "a369977c",
+      "fileHash": "9022993e",
       "needsInterop": false
     },
     "element-plus": {
       "src": "../../element-plus/es/index.mjs",
       "file": "element-plus.js",
-      "fileHash": "58b53aee",
+      "fileHash": "ed152bc7",
       "needsInterop": false
     },
     "fabric": {
       "src": "../../fabric/dist/index.min.mjs",
       "file": "fabric.js",
-      "fileHash": "a1369625",
+      "fileHash": "14f1f947",
       "needsInterop": false
     },
     "pinia": {
       "src": "../../pinia/dist/pinia.mjs",
       "file": "pinia.js",
-      "fileHash": "20bc890b",
+      "fileHash": "ecda6472",
       "needsInterop": false
     },
     "vue-router": {
       "src": "../../vue-router/dist/vue-router.mjs",
       "file": "vue-router.js",
-      "fileHash": "017f450e",
+      "fileHash": "4a9d54f8",
       "needsInterop": false
     },
     "vue": {
       "src": "../../vue/dist/vue.runtime.esm-bundler.js",
       "file": "vue.js",
-      "fileHash": "70ae02ae",
+      "fileHash": "4a46bbb6",
       "needsInterop": false
     }
   },