fix: improve chat UI streaming feedback and fix test image path

- Add loading spinner with 'Thinking...' text during streaming when
  content is not yet available
- Fix :key binding to use message.id instead of message.content
- Remove unnecessary TypeScript cast in file reader handler
- Move test image into testdata/ directory for proper test organization
- Remove t.Skip and simplify TestSummarizeChat test message

frontend/public/pages/chats.html

@@ -3,7 +3,7 @@
   <div
     class="h-dvh pt-16 flex flex-col-reverse pb-36 overflow-scroll mx-auto px-4 md:px-6 max-w-6xl"
   >
-    <template x-for="message in currentChatMessages" :key="message.content">
+    <template x-for="message in currentChatMessages" :key="message.id">
       <div
         :class="['mb-4', message.role === 'user' ? 'text-right' : 'text-left']"
       >
@@ -50,8 +50,20 @@
 
           <hr x-show="message.thinking" class="my-2 border-primary-400/50" />
 
+          <!-- Loading Spinner (Streaming with no content yet) -->
+          <div
+            x-show="message.role === 'assistant' && message.status === 'streaming' && !message.thinking && !message.content"
+            class="flex items-center gap-2 py-1"
+          >
+            <div
+              class="h-4 w-4 animate-spin rounded-full border-2 border-primary-500 border-t-transparent"
+            ></div>
+            <span class="text-xs text-primary-600">Thinking...</span>
+          </div>
+
           <!-- Main Content -->
           <div
+            x-show="message.content || message.status !== 'streaming'"
             class="prose max-w-none text-sm prose-p:my-1 prose-headings:my-2 prose-ul:my-1 prose-ol:my-1 prose-pre:p-0"
             x-html="renderMarkdown(message.content)"
           ></div>
@@ -224,7 +236,7 @@
             @change="
               Array.from($event.target.files).forEach(file => {
                 const reader = new FileReader();
-                reader.onload = (e) => { selectedImages.push(e.target.result as string); };
+                reader.onload = (e) => { selectedImages.push(e.target.result); };
                 reader.readAsDataURL(file);
               });
               $event.target.value = '';