Fix assistant message text truncating with "…" instead of wrapping

Two contributing layout issues in the chat message bubble:

1. MessageRow's content VStack (icon + text HStack) had no
   .frame(maxWidth: .infinity), so it sized to its content's ideal
   width instead of the space actually available.

2. swift-markdown-ui renders paragraphs with mixed inline styling
   (bold/italic runs next to plain text) as concatenated Text(+)
   segments, which on macOS report their ideal unwrapped single-line
   size for height purposes instead of wrapping — truncating with "…"
   regardless of window width. Plain single-style paragraphs (a single
   Text) weren't affected, which is why some lines wrapped fine and
   others didn't.

Fixed by adding .frame(maxWidth: .infinity, alignment: .leading) to
the MessageRow content stack, and .fixedSize(horizontal: false,
vertical: true) to the markdown paragraph label so height is
recomputed for the width actually given instead of the ideal width.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

oAI/Views/Main/MessageRow.swift

@@ -185,6 +185,7 @@ struct MessageRow: View {
                     .foregroundColor(.oaiSecondary)
                 }
             }
+            .frame(maxWidth: .infinity, alignment: .leading)
         }
         .padding(16)
         .background(Color.messageBackground(for: message.role))