Focus note editor blank space

[ComputelessComputer]

Add a shared trailing body-line command and wire blank note-surface clicks to it.

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Note

Low Risk
Localized note-editing UX and focus/selection behavior with new unit tests; no auth, persistence, or API contract changes beyond the editor command surface.

Overview
Clicking empty padding around the session note editor now moves the caret to the trailing body line (reusing or inserting an empty paragraph) instead of a generic focus.

The shared ProseMirror helper focusTrailingEmptyLine is exported and exposed as focusAtTrailingEmptyLine on note editor commands. NoteInput switches the scroll container from onClick to onMouseDown, ignores non–left-clicks and clicks inside .ProseMirror, and routes the rest to that command. The in-editor trailing-empty-line plugin still uses the same helper, including title-only notes that need a new body paragraph.

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