fix(web): prioritize model in mobile header

[sam-saffron-jarvis]

Summary

• keep the mobile header on 1 physical row instead of wrapping controls beneath long conversation titles
• prioritize the full runtime model control while clamping the conversation title to a single ellipsized line
• omit the project subtitle below 768px; project context remains available in the sidebar
• preserve the existing menu, plan/status, and diff actions
• add a stylesheet regression test for the model-priority mobile layout

Tests

• npm test — 405 tests passed
• npm run lint
• npm run typecheck
• npm run build