Stabilize live heading collapse

[sufyanaser]

What changed

• moved heading collapse state into a ProseMirror plugin and node decorations
• preserved collapsed sections across editor transactions and document position mapping
• reset collapse state safely when switching notes
• kept toolbar and inline heading controls synchronized
• removed the global DOM mutation observer fallback

Root cause

The previous implementation wrote unmanaged attributes directly into ProseMirror-owned editable DOM. ProseMirror reconciled those attributes during live editing, so collapse only appeared reliable after locking the note and could jump when transactions rebuilt the DOM.

User impact

Heading sections now collapse and expand immediately while a note is unlocked, without changing saved note HTML or shifting the heading viewport position.

Validation

• lint: passed with zero warnings
• TypeScript check: passed
• tests: 62/62 passed
• production build: passed
• npm audit: 0 vulnerabilities
• Electron native-module smoke test: passed
• real Electron/CDP collapse-expand test: passed; heading top remained 1963.125 px before and after toggle