feat: make sidebar panels resizable (#2094)

- Replaces #1870, which replaced #1682, #1677

This mostly uses the code from there and addresses the comments I've
left. I didn't change the code much.

## Launch Checklist

Makes the sidebar panel resizable

<img width="707" height="511" alt="image"
src="https://github.com/user-attachments/assets/d010df38-faac-441c-bf7f-a4960e471c61"
/>


 - [x] Briefly describe the changes in this PR.
 - [x] Link to related issues.
- [x] Include before/after visuals or gifs if this PR includes visual
changes.
 - [x] Write tests for all new functionality.
 - [x] Add an entry to `CHANGELOG.md` under the `## main` section.

---------

Co-authored-by: Bart Louwers <bart.louwers@gmail.com>

src/locales/ja/translation.json

@@ -54,6 +54,8 @@
   "Delete expression": "式を削除",
   "Delete filter block": "フィルタブロックを削除",
   "Deuteranopia filter": "緑色盲フィルタ",
+  "Drag to resize the layer list": "ドラッグしてレイヤーリストのサイズを変更",
+  "Drag to resize the sidebar": "ドラッグしてサイドバーのサイズを変更",
   "Duplicate": "複製",
   "Encoding": "エンコーディング",
   "Enter URL...": "URLを入力",