feat: render task notifications as styled cards instead of raw XML

[Psypeal]

Problem

When Claude Code runs background tasks (run_in_background), completion notifications are injected as <task-notification> XML blocks in user messages. These display as raw XML in the chat view, which is confusing and hard to read.

Example of what users currently see:

<task-notification><task-id>bc8lpoiyk</task-id><tool-use-id>toolu_01FY9VUcR6...</tool-use-id>
<output-file>/tmp/claude-1000/.../bc8lpoiyk.output</output-file>
<status>completed</status><summary>Background command "Run mediation analyses" completed (exit code 0)</summary></task-notification>
Read the output file to retrieve the result: /tmp/claude-1000/.../bc8lpoiyk.output

Proposed Solution

1. Parse <task-notification> XML blocks from user message content
2. Strip the raw XML and trailing "Read the output file..." instruction from display text
3. Render as compact styled cards showing: status icon (check/x), command name, exit code, output filename

Files affected

• src/shared/utils/contentSanitizer.ts — add parser + strip pattern
• src/renderer/components/chat/UserChatGroup.tsx — render parsed notifications as cards