Fix multi-plate 3MF metadata extraction (Issue #92)

When exporting individual plates from a multi-plate 3MF in Bambu Studio,
all uploaded archives showed plate 1's name and thumbnail regardless of
which plate was actually exported.

Root cause: The 3MF parser used an incorrect XPath lookup (plate_idx
attribute) and didn't extract the plate index from slice_info.config
metadata.

Changes:
- Extract plate index from <metadata key="index" value="N"/> in
  slice_info.config
- Remove incorrect plate[@plate_idx='N'] XPath lookup that doesn't
  work for single-plate exports
- Set self.plate_number from extracted index so _extract_thumbnail()
  uses the correct plate thumbnail (e.g., plate_5.png instead of
  plate_1.png)
- Append " - Plate N" to print_name when plate index > 1 to
  distinguish multi-plate exports
- Add 7 unit tests for plate index extraction and print_name
  enhancement

static/index.html

@@ -23,7 +23,7 @@
 
     <!-- Splash screens for iOS -->
     <link rel="apple-touch-startup-image" href="/img/android-chrome-512x512.png" />
-    <script type="module" crossorigin src="/assets/index-CDWq7hKx.js"></script>
+    <script type="module" crossorigin src="/assets/index-CVMDsKi7.js"></script>
     <link rel="stylesheet" crossorigin href="/assets/index-Dzh7xD3q.css">
   </head>
   <body>