fix: keep the opened note stationary during drags

[fjbarrett]

Summary

Opened notes now stay stationary during drags. A fixed cover replaces the movable sheet while retaining the native card zoom, rounded editor, text insets, and × close button. Body text remains scrollable.

Sign-in keeps its sheet presentation and waits for an open editor to finish closing when authentication expires.

Test plan

• Regenerated projects; KeepMac, signed iPhone, and SE simulator builds passed.
• Recorded native card opening, slow downward, title-area, and edge drags, and closing. The title, close control, color control, and surface remain fixed while body text scrolls; the native zoom has no nil-source fallback warning.
• Verified new-note editing and autosave. Simulated a 401 during editing: the editor closes, sign-in appears, and the durable draft saves after recovery.
• Existing native and web CI checks passed. No new tests for this presentation change.
• Physical SE build 20260905.380 is installed and its version verified. The retained Xcode project matches the merged source. Launch succeeded after unlock; live PID 57887 matches the installed build 380 container.

Model: GPT-6
Co-Authored-By: Codex noreply@openai.com