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>

docs/openapi.json

@@ -10,7 +10,7 @@
     "license": {
       "name": "Apache-2.0"
     },
-    "version": "0.3.0"
+    "version": "0.3.1"
   },
   "servers": [
     {