REDMINE-21205

Allows moving sections and content elements to different positions
via a dialog that displays the entry outline with clickable targets.

REDMINE-21205

Add handleMove to textBlock content element type to allow moving
selected text ranges (not just the entire element) to a different
section. When handleMove is defined on a content element type,
MoveContentElementMenuItem shows "Move selection..." label and
delegates to handleMove instead of entry.moveContentElement.

The MOVE_TO command handler in EditableText computes the bounds of
the current selection and reuses existing postMoveContentElementMessage
infrastructure.

Extract computeBounds from Selection.js to shared module to remove
duplication.

Redmine: 21205

Display a helpful message when a storyline has no chapters, so users
understand why no move destinations are available.

Redmine: 21205

Adds clickable insert target to empty chapters in the move destination
dialog. Extracts shared tooltip and insert line styles to a reusable
CSS module.

REDMINE-21205

When moving a content element via the menu action, the preview now
scrolls to show the target location once the move completes. Uses
'nearStart' alignment when moving to beginning and 'nearEnd' when
moving to end of section.

REDMINE-21205