fix: constrain chat assistant to 384px on desktop and remove dark background (#224)

## Summary

- Constrains the chat assistant floating input to `max-width: 384px` and
centers it on desktop (`>=640px`), overriding Mintlify's new full-width
default
- Removes the dark background behind the chat area by making the sticky
container and its `::before` pseudo-elements transparent
- Mobile behavior (8px edge-to-edge overflow) is unchanged

## Context

Mintlify pushed a platform update that made the chat assistant `w-full`
(confirmed on their Palm theme starter). This is not caused by our CSS —
we need to override their new default.

## Test plan

- [x] Desktop: chat bar is ~384px wide, centered, no dark background
behind it
- [x] Mobile: chat bar extends 8px past content edges as before
- [x] Light and dark mode both look correct

Made with [Cursor](https://cursor.com)

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: patcapulong

mintlify/style.css

@@ -587,6 +587,16 @@ html.dark .navbar-link svg {
   transition: none !important;
 }
 
+/* Remove dark background behind chat area (sticky container + pseudo-elements) */
+div:has(> .chat-assistant-floating-input) {
+  background: transparent !important;
+}
+
+div:has(> .chat-assistant-floating-input)::before,
+.chat-assistant-floating-input > div::before {
+  background: transparent !important;
+}
+
 /* Mobile assistant bar placeholder - make transparent so it doesn't cover content */
 #assistant-bar-placeholder {
   background: transparent !important;
@@ -605,6 +615,14 @@ html.dark .navbar-link svg {
   }
 }
 
+/* Desktop: constrain chat bar to 384px and center */
+@media (min-width: 640px) {
+  .chat-assistant-floating-input {
+    max-width: 384px !important;
+    margin: 0 auto !important;
+  }
+}
+
 /* Container wrapper - glassy blur effect */
 .chat-assistant-floating-input > div > div {
   background: var(--ls-white-70) !important;