feat: modernize chat UI and rename handoff to task

- Remove agent message bubbles, keep user bubbles (right-aligned)
- Rename "Handoffs" to "Tasks" with ListChecks icon in sidebar
- Move model picker inside composer, add renderFooter to ChatComposer SDK
- Make project sections collapsible with hover-only chevrons
- Remove divider between chat and composer
- Update model picker chevron to flip on open/close
- Replace all user-visible "handoff" strings with "task" across frontend

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/packages/inspector/index.html

@@ -1510,10 +1510,11 @@
       }
 
       .message.assistant .message-content {
-        background: var(--surface);
-        border: 1px solid var(--border);
+        background: none;
+        border: none;
         color: var(--text-secondary);
-        border-bottom-left-radius: 4px;
+        border-radius: 0;
+        padding: 0 16px;
       }
 
       .message.system .avatar {