[Requests and ideas] PR review comments: add rich text editor with image drag-and-drop (parity with github.com)

**Mood:** 😊
**Category:** Requests and ideas

## Summary

The PR review comment editor in GitHub Desktop App is plain-text only. On github.com, the review comment editor supports a rich toolbar (bold, italic, lists, headings, code blocks, etc.) and drag-and-drop / paste image upload. Neither capability is available in the app.

## What I'd expect

The review comment pane should match github.com's editor capabilities:
- **Image upload** -- drag-and-drop or paste images directly into review comments (inline and top-level)
- **Rich text toolbar** -- formatting controls (bold, italic, lists, code, etc.) like the web editor

## Why this matters

Reviewers frequently need to attach screenshots for visual context (UI regressions, layout issues, annotated mockups). Without image support, the workaround is to switch to the browser, which defeats the purpose of reviewing in the app.

## Current behavior

The review pane shows a plain textarea with no formatting toolbar and no image upload support.

## Expected behavior (github.com reference)

The web editor includes a full Markdown toolbar and image drag-and-drop.

## Environment

---
| Field | Value |
| --- | --- |
| App version | 0.2.27 |
| OS | macOS 26.3.1 |
| Theme | GitHub |
| Path | /inbox |
| Tenure | Day 1 |

从 /inbox 的审查评论面板开始，并将其纯文本 textarea 与 issue 中描述的 github.com 编辑器行为进行比较。当审查评论支持所请求的富文本控件，以及对内联评论和顶层评论都支持拖放或粘贴图片时，即视为完成。

由索引模型根据 Issue 内容生成。