fix: preserve file_path on comments added via web UI

[tomasz-tomczyk]

Summary

• document-renderer.js was passing null as the filePath argument to renderBlock for single-file reviews, so every block's dataset.filePath was set to an empty string
• When a comment was submitted, submitNewComment sent file_path: null to the server
• Comments with nil file_path are filtered out by the export endpoints (multi_file_comments_json), causing them to silently disappear from "Get prompt" and export responses

Fix

Store f.path as ctx.singleFilePath when the init event arrives with a single file, then pass it through to renderBlock instead of hardcoded null.

Fixes #6