Open rewind in an anchored dropdown menu

Replace the full-screen rewind sheet with a dropdown anchored to the
trigger, with a muted warning header and inline pending state on the
chosen action. Also realign the user message action row so timestamp,
rewind, and copy share a single 24px baseline — the copy button was
inheriting assistant-footer paddings that pushed the row taller than
the icons.

packages/app/src/components/message.tsx

@@ -409,8 +409,14 @@ const userMessageStylesheet = StyleSheet.create((theme) => ({
     fontSize: theme.fontSize.sm,
   },
   copyButton: {
-    padding: theme.spacing[1],
+    alignSelf: "center",
+    width: 24,
+    height: 24,
+    padding: 0,
+    margin: 0,
     marginRight: -theme.spacing[1],
+    alignItems: "center",
+    justifyContent: "center",
   },
   trailingRow: {
     alignSelf: "flex-end",