fix(MessageList): set width on message list scroll container (#3077)

### 🎯 Goal

`.str-chat__message-list-scroll` is a flex child inside
`.str-chat__message-list` (which uses `display: flex; justify-content:
center`), but it had no `width` set — so it shrink-wrapped its content
instead of filling the available space up to its `max-width`.

### 🛠 Implementation details

- Added `width: 100%` to `.str-chat__message-list-scroll` in
`MessageList.scss` so it expands to fill the parent, capped by the
existing `max-width`
- Removed the now-redundant `width: 100%` override from the vite example
app's `index.scss`

### 🎨 UI Changes

No visual change in the vite example app (which already had the
workaround). Other consumers that didn't apply their own override will
now see the message list correctly fill the available width.

Author: oliverlaz

examples/vite/src/index.scss

@@ -213,10 +213,6 @@ body {
     max-width: none;
   }
 
-  .str-chat__message-list-scroll {
-    width: 100%;
-  }
-
   .str-chat__list,
   .str-chat__virtual-list {
     display: block;

src/components/MessageList/styling/MessageList.scss

@@ -30,6 +30,7 @@
 
   .str-chat__message-list-scroll {
     @include utils.message-list-spacing;
+    width: 100%;
     max-width: var(--str-chat__message-list-scroll-max-width);
     .str-chat__ul {
       list-style: none;