M7 #13 (pass-2): suppress empty assistant bubble during reasoning-only frames

Pass-2 turned up a ghost-message UX bug we missed in pass-1: every
"Thinking…" reasoning disclosure had an empty gray bubble next to
it. Happens because assistant messages exist momentarily in a
reasoning-only state (chunks of thinking text arrive before any
primary content), and the bubble path always rendered its padded
background regardless of content.

Gate the bubble render on non-empty content for assistant messages.
User bubbles still always render (the user explicitly submitted
content and saw it land — suppressing it on trim-empty would be
surprising). `trimmingCharacters(in: .whitespacesAndNewlines)` so
purely-whitespace assistant frames also don't render a bubble.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scarf/Scarf iOS/Chat/ChatView.swift

@@ -672,7 +672,18 @@ private struct MessageBubble: View {
                     if message.hasReasoning, let r = message.reasoning, !r.isEmpty {
                         ReasoningDisclosure(reasoning: r)
                     }
-                    bubbleContent
+                    // Only render the bubble when there's actual text
+                    // to show. Assistant messages can exist in a
+                    // "reasoning-only" or "tool-calls-only" state
+                    // while the agent is thinking / invoking tools —
+                    // rendering an empty gray bubble next to every
+                    // "Thinking…" disclosure looked like a ghost
+                    // message. User bubbles we always render (the
+                    // user explicitly submitted content, even if
+                    // it's just whitespace, they saw it land).
+                    if message.isUser || !message.content.trimmingCharacters(in: .whitespacesAndNewlines).isEmpty {
+                        bubbleContent
+                    }
                     if !message.toolCalls.isEmpty {
                         VStack(alignment: .leading, spacing: 6) {
                             ForEach(message.toolCalls) { call in