[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 |

Start at the /inbox review comment pane and compare its plain textarea with the github.com editor behavior described in the issue. Done means review comments support the requested rich-text controls and image drag-and-drop or paste for both inline and top-level comments.

索引モデルが issue の本文から書いたものです。