fix: make Writers Room headers collapsible and stabilize Save

[atomantic]

Summary

• Add a remembered header disclosure that hides catalog cast and secondary writing controls, while keeping the title, Save, and library access available.
• Preserve the mounted editor and unsaved prose when collapsing or expanding; give the editor the remaining available height.
• Stabilize Save with a fixed width, explicit state label, and busy status. Let long titles yield space on narrow phones.

Test plan

• Passed 18 WorkEditor interaction and performance tests, including collapse/expand preservation, typing during save, failure, retry, and saved-state transitions.
• Client production build and lint of changed files passed.
• Optional Claude read-only review reported no issues but returned a malformed verdict, recorded as inconclusive under the configured one-round policy.
• Exact rendering on the affected mobile browser remains unverified.