fix(frontend): show Thinking… placeholder between Send and first SSE event

Closes #127

Before this change, the live streaming area in Agent.tsx only rendered
when streamingText was non-empty or toolCalls had at least one entry.
The gap between handleSubmit setting status="streaming" and the first
SSE event arriving left the chat area visually blank, making the UI
look unresponsive on slow networks or long agent "thinking" time.

Add a Thinking… placeholder that renders exactly during that gap
(status === "streaming" && !streamingText && toolCalls.length === 0)
and is implicitly hidden once the first SSE event populates either
streaming state.

Author: warren618

frontend/src/pages/Agent.tsx

@@ -472,6 +472,17 @@ export function Agent() {
             );
           })}
 
+          {/* Pre-stream placeholder: visible after Send, before first SSE event */}
+          {status === "streaming" && !streamingText && toolCalls.length === 0 && (
+            <div className="flex gap-3">
+              <AgentAvatar />
+              <div className="flex-1 min-w-0 flex items-center gap-2 text-xs text-muted-foreground pt-1">
+                <Loader2 className="h-3 w-3 animate-spin text-primary shrink-0" />
+                <span>Thinking…</span>
+              </div>
+            </div>
+          )}
+
           {/* Live streaming area: text + tool status */}
           {(streamingText || (status === "streaming" && toolCalls.length > 0)) && (
             <div className="flex gap-3">