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>

package-lock.json

@@ -58,6 +58,7 @@
         "react-i18next": "^17.0.15",
         "react-icons": "^5.7.0",
         "react-markdown": "^10.1.0",
+        "react-resizable-panels": "^4.11.0",
         "reconnecting-websocket": "^4.4.0",
         "slugify": "^1.6.9",
         "string-hash": "^1.1.3",
@@ -10758,6 +10759,16 @@
         "react": ">=18"
       }
     },
+    "node_modules/react-resizable-panels": {
+      "version": "4.11.0",
+      "resolved": "https://registry.npmjs.org/react-resizable-panels/-/react-resizable-panels-4.11.0.tgz",
+      "integrity": "sha512-LPk/AkFDGkg7SsbOyL93ojrE6E7lhrxxDwnYNjfmnSeI6BE7Sje6dB24PXgZk8DeugdeXNk1LO+ohRqIjhxiLw==",
+      "license": "MIT",
+      "peerDependencies": {
+        "react": "^18.0.0 || ^19.0.0",
+        "react-dom": "^18.0.0 || ^19.0.0"
+      }
+    },
     "node_modules/reactcss": {
       "version": "1.2.3",
       "resolved": "https://registry.npmjs.org/reactcss/-/reactcss-1.2.3.tgz",