feat: Compact transparent tool cards with grouped assistant messages

- Redesign all tool cards (Edit, Read, Bash, Grep, Glob, Write) to be
  compact single-line headers with inline key info and toggle buttons
- Make cards and bubbles fully transparent with subtle color tints
- Remove borders, use only left accent bar per card type
- Color-code numbers: red for removals, green for additions/counts
- Simplify ToolResultBlock to render content directly without toggle
- Group consecutive assistant messages, showing header only on first
- Remove borders from assistant and user message bubbles

frontend/src/components/transcript-debug/ChatContainer.vue

@@ -208,11 +208,12 @@ function formatDuration(start: string, end: string): string {
     </div>
     <div ref="scrollContainer" class="messages-scroll">
       <div
-        v-for="msg in conversation.messages"
+        v-for="(msg, idx) in conversation.messages"
         :key="msg.uuid"
         :class="['message-wrapper', {
           resolved: resolvedUuids.has(msg.uuid),
-          selected: selectedUuids.has(msg.uuid)
+          selected: selectedUuids.has(msg.uuid),
+          'assistant-continuation': msg.kind === 'assistant' && idx > 0 && conversation.messages[idx - 1].kind === 'assistant'
         }]"
       >
         <!-- Select checkbox -->
@@ -227,7 +228,7 @@ function formatDuration(start: string, end: string): string {
           </svg>
         </button>
 
-        <div :class="['message-content', { selectable: selectMode && msg.kind !== 'progress' }]" @click="selectMode && msg.kind !== 'progress' ? toggleSelect(msg.uuid) : undefined">
+        <div :class="['message-content', { selectable: selectMode && msg.kind !== 'progress' }]" @click="selectMode && msg.kind !== 'progress' && toggleSelect(msg.uuid)">
           <UserMessageBubble
             v-if="msg.kind === 'user'"
             :message="msg"
@@ -235,6 +236,7 @@ function formatDuration(start: string, end: string): string {
           <AssistantMessageBubble
             v-else-if="msg.kind === 'assistant'"
             :message="msg"
+            :show-header="!(idx > 0 && conversation.messages[idx - 1].kind === 'assistant')"
           />
           <ProgressEvent
             v-else-if="msg.kind === 'progress'"
@@ -448,6 +450,11 @@ function formatDuration(start: string, end: string): string {
   padding: 0.25rem 0.4rem;
 }
 
+/* Consecutive assistant messages: tighter spacing */
+.message-wrapper.assistant-continuation {
+  margin-top: -0.5rem;
+}
+
 /* Messages that replaced an optimistic one: smooth settle instead of bounce */
 .message-wrapper.resolved {
   animation: resolveIn 0.3s ease-out;