feat: add note lock feature to prevent editing

[j4rviscmd]

Summary

• Add lock/unlock toggle for notes that prevents content editing while still allowing text selection for copying
• Uses a ProseMirror filterTransaction plugin (readOnlyGuard) to block all editing transactions when locked, keeping contentEditable="true" so WebKit text selection works
• Hides editing UI elements (drag handles, side menu, formatting toolbar) via CSS [data-locked] attribute
• Hides text caret with caret-color: transparent when locked for a read-only feel

Changes

Backend (Rust)

• Add is_locked column to SQLite notes table with migration
• Add toggle_lock Tauri command
• Extract NOTE_COLUMNS constant to DRY up SELECT statements

Frontend (TypeScript/React)

• Add readOnlyGuard ProseMirror extension that blocks doc-changing transactions
• Add data-locked attribute to editor wrapper for CSS targeting
• Show FileLock icon in sidebar for locked notes
• Show lock indicator in SaveStatusIndicator
• Reset lock state on note switch to prevent stale UI
• Remove redundant locked guard from handleChange (plugin handles it)

CSS

• Hide drag handles, side menu, add-block button, suggestion menu when locked
• Override cursor to default and hide caret when locked

Test plan

• Lock a note via context menu → verify editing is blocked
• Lock a note → verify text selection still works for copying
• Lock a note → verify drag handles, side menu, formatting toolbar are hidden
• Lock a note → verify text caret is hidden (no blinking I-beam)
• Unlock a note → verify editing is restored
• Switch from locked note to unlocked note → verify lock state resets
• Verify FileLock icon shows in sidebar for locked notes
• Verify lock indicator shows in header save status area

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