Add agent conversation screen

[klopez4212]

Summary

• Split from Add agent task conversation foundation #1323.
• Add the dedicated agent conversation screen and screen helper layer.
• Add persisted shell state for selecting/hiding/updating focused conversations.
• Add the small shared UI support needed by the screen: animated header title and composer notices.

Stack

1. Add agent conversation runtime foundation #1384
2. Add agent conversation domain model #1385
3. Add marker-aware thread behavior #1386
4. This PR
5. kennylopez-agent-tasks-navigation-integration

Tests

• Pre-commit hooks
• Pre-push hooks
• git diff --check 325b3fdb0d74047d6fc54bfda7c02a008af0941c..HEAD
• cd desktop && pnpm check:file-sizes && ./node_modules/.bin/tsc --noEmit