Fix Chinese character encoding in tooltip display

Fixed UTF-8 decoding issue where Chinese characters were displayed as
garbled text (e.g., 预计 instead of 预计) in the top-10 predictions
tooltip.

Root cause: JavaScript's atob() function only supports Latin-1 encoding,
not UTF-8. When decoding base64-encoded JSON containing Chinese characters,
each UTF-8 byte was incorrectly interpreted as a separate Latin-1 character.

Solution: Implemented proper UTF-8 decoding by:
1. Using atob() to decode base64 to binary string
2. Converting binary string to Uint8Array
3. Using TextDecoder('utf-8') to correctly decode UTF-8 bytes

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

visualization/html_generator.py

@@ -773,8 +773,13 @@ def generate_comparison_html(
                 function formatTopkColumn(topkBase64, modelName, titleClass) {{
                     if (!topkBase64) return '<div class="topk-column"><div class="topk-title ' + titleClass + '">' + modelName + '</div><div class="topk-list">N/A</div></div>';
                     try {{
-                        // Decode base64 to JSON string
-                        const topkJson = atob(topkBase64);
+                        // Decode base64 to UTF-8 string (atob() doesn't support UTF-8, need proper decoding)
+                        const binaryString = atob(topkBase64);
+                        const bytes = new Uint8Array(binaryString.length);
+                        for (let i = 0; i < binaryString.length; i++) {{
+                            bytes[i] = binaryString.charCodeAt(i);
+                        }}
+                        const topkJson = new TextDecoder('utf-8').decode(bytes);
                         const data = JSON.parse(topkJson);
                         const [actualId, rank, topkList] = data;
                         let html = '<div class="topk-column">';