feat: enhance HTML export with syntax highlighting, diff view, and An…

[Harryoung]

…thropic brand styling

• Fix Unicode encoding: use ensure_ascii=False in json.dumps so Chinese text renders correctly instead of showing escape sequences
• Extract tool_use blocks from assistant messages as separate structured entries (previously they were flattened into plain text strings because the JSONL stores tool calls inside assistant content arrays, not as standalone entries)
• Extract tool_result from user messages in detailed mode
• Write tool: render file path header + syntax-highlighted code block using highlight.js with language detection from file extension
• Edit tool: render old_string/new_string as GitHub-style diff view with red (removed) and green (added) lines
• Read/Bash/Grep/Glob tools: compact parameter display
• Apply Anthropic brand design system: dark header (#141413), warm background (#faf9f5), Poppins headings, Lora body text, brand accent colors for message borders (blue=Human, green=Claude, orange=Tool)