style: add expand button positioning and styles

packages/ui/src/styles/messaging/prompt-input.css

@@ -27,6 +27,7 @@
   flex: 1 1 auto;
   height: 100%;
   min-width: 0;
+  transition: height 0.25s ease;
 }
 
 .prompt-input-field {
@@ -105,6 +106,40 @@
   cursor: not-allowed;
 }
 
+.prompt-expand-top {
+  position: absolute;
+  top: 0.3rem;
+  right: 3.5rem;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  z-index: 2;
+}
+
+.prompt-expand-button {
+  @apply w-9 h-9 flex items-center justify-center rounded-md;
+  color: var(--text-muted);
+  background-color: rgba(15, 23, 42, 0.04);
+  transition: background-color 0.15s ease, color 0.15s ease;
+  padding: 0;
+}
+
+.prompt-expand-button:hover:not(:disabled) {
+  background-color: var(--surface-secondary);
+  color: var(--text-primary);
+}
+
+.prompt-expand-button:active:not(:disabled) {
+  background-color: var(--accent-primary);
+  color: var(--text-inverted);
+  transform: scale(0.95);
+}
+
+.prompt-expand-button:disabled {
+  opacity: 0.4;
+  cursor: not-allowed;
+}
+
 .prompt-overlay-text {
   display: inline-flex;
   align-items: center;