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/UserMessageBubble.vue

@@ -33,10 +33,10 @@ function formatTime(ts: string): string {
 
 <style scoped>
 .user-bubble {
-  background: rgba(99, 102, 241, 0.04);
-  backdrop-filter: blur(6px);
-  -webkit-backdrop-filter: blur(6px);
-  border: 1px solid rgba(99, 102, 241, 0.12);
+  background: transparent;
+  backdrop-filter: blur(12px);
+  -webkit-backdrop-filter: blur(12px);
+  border: none;
   border-radius: 12px;
   padding: 0.75rem 1rem;
   margin-left: 2rem;
@@ -51,9 +51,9 @@ function formatTime(ts: string): string {
 /* Optimistic / sending state */
 .user-bubble.optimistic {
   opacity: 0.7;
-  border-color: rgba(99, 102, 241, 0.12);
+  border-color: rgba(99, 102, 241, 0.08);
   border-style: dashed;
-  background: rgba(99, 102, 241, 0.04);
+  background: transparent;
 }
 
 .sending-badge {