Add oekaki drawing flow in post/reply with Tegaki, using auto-upload

[tomcasaburi]

5chan currently lacks an integrated oekaki posting flow like 4chan’s drawing boards, so users can’t draw and attach artwork directly from the post UI. APK/Electron already support auto-upload via the existing Catbox plugin, but there is no draw→finish→upload path that fills comment.link automatically. Web users also need a clear fallback because browser CORS prevents auto-upload, so they still have to paste a direct image URL manually.

Use tegaki-js (desuwa/tegaki) and add a new drawing row in both post form and reply modal with size inputs (width x height, default 400 x 400) plus Draw/Clear (later Edit/Clear). Draw opens Tegaki fullscreen; Close returns without changing the link; Finish exports and uploads on APK/Electron through the existing auto-upload plugin, then auto-fills comment.link with the direct URL. After first finish, Edit reopens the drawing and re-finishing uploads again and refreshes the link, while Clear removes drawing state and clears the auto-filled link.