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>

e2e/sidebar-resize.spec.ts

@@ -0,0 +1,27 @@
+import { beforeEach, describe, test } from "./utils/fixtures";
+import { MaputnikDriver } from "./maputnik-driver";
+
+describe("sidebar resize", () => {
+  const { given, get, when, then } = new MaputnikDriver();
+
+  beforeEach(async () => {
+    await given.setupMockBackedResponses();
+    await when.setStyle("layer");
+  });
+
+  test("dragging the outer handle widens the sidebar", async () => {
+    const initialWidth = await get.elementWidth("sidebar-panel").get();
+
+    await when.dragBy("sidebar-resize-handle", 100);
+
+    await then(get.elementWidth("sidebar-panel")).shouldBeGreaterThan(initialWidth + 50);
+  });
+
+  test("dragging the inner handle widens the layer list", async () => {
+    const initialWidth = await get.elementWidth("layer-list-panel").get();
+
+    await when.dragBy("inner-resize-handle", 50);
+
+    await then(get.elementWidth("layer-list-panel")).shouldBeGreaterThan(initialWidth + 20);
+  });
+});