feat: add automatic CSS formatting with Prettier

Integrates Prettier into the stylesheet store for automatic CSS formatting:
- Installs prettier v3.7.4 with postcss plugin
- Implements formatContent() function using Prettier API
- Auto-formats CSS after 500ms of inactivity (debounced)
- Prevents infinite loops with isFormatting flag
- Ensures consistent indentation and line breaks
- Cleans up extra blank lines and formatting issues

This ensures the CSS in the Code tab stays clean and properly formatted
after reactive edits from TextSettings and PageSettings panels.

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

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

package-lock.json

@@ -12,6 +12,7 @@
         "highlight.js": "^11.11.1",
         "pagedjs": "^0.4.3",
         "pinia": "^3.0.4",
+        "prettier": "^3.7.4",
         "vue": "^3.5.24"
       },
       "devDependencies": {
@@ -1888,6 +1889,21 @@
         "node": "^10 || ^12 || >=14"
       }
     },
+    "node_modules/prettier": {
+      "version": "3.7.4",
+      "resolved": "https://registry.npmjs.org/prettier/-/prettier-3.7.4.tgz",
+      "integrity": "sha512-v6UNi1+3hSlVvv8fSaoUbggEM5VErKmmpGA7Pl3HF8V6uKY7rvClBOJlH6yNwQtfTueNkGVpOv/mtWL9L4bgRA==",
+      "license": "MIT",
+      "bin": {
+        "prettier": "bin/prettier.cjs"
+      },
+      "engines": {
+        "node": ">=14"
+      },
+      "funding": {
+        "url": "https://github.com/prettier/prettier?sponsor=1"
+      }
+    },
     "node_modules/readdirp": {
       "version": "4.1.2",
       "resolved": "https://registry.npmjs.org/readdirp/-/readdirp-4.1.2.tgz",