feat: iframe-based PagedJS preview with reactive CSS editor

- Isolate PagedJS in iframe to avoid DOM/CSS conflicts
- Add EditorPanel for global CSS controls
- Add StylesheetViewer with highlight.js syntax highlighting
- Add ElementPopup for element-specific CSS editing
- CSS modifications update preview reactively
- Support px/rem/em units

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

package-lock.json

@@ -8,6 +8,7 @@
       "name": "geoproject",
       "version": "0.0.0",
       "dependencies": {
+        "highlight.js": "^11.11.1",
         "pagedjs": "^0.4.3",
         "vue": "^3.5.24"
       },
@@ -1182,6 +1183,15 @@
         "node": "^8.16.0 || ^10.6.0 || >=11.0.0"
       }
     },
+    "node_modules/highlight.js": {
+      "version": "11.11.1",
+      "resolved": "https://registry.npmjs.org/highlight.js/-/highlight.js-11.11.1.tgz",
+      "integrity": "sha512-Xwwo44whKBVCYoliBQwaPvtd/2tYFkRQtXDWj1nackaV2JPXx3L0+Jvd8/qCJ2p+ML0/XVkJ2q+Mr+UVdpJK5w==",
+      "license": "BSD-3-Clause",
+      "engines": {
+        "node": ">=12.0.0"
+      }
+    },
     "node_modules/magic-string": {
       "version": "0.30.21",
       "resolved": "https://registry.npmjs.org/magic-string/-/magic-string-0.30.21.tgz",