A trigger->replacement library (core/shortcuts.py, a close port of v4's
apply_prompt_shortcuts/resolve_variables), a Settings section to manage it,
autocomplete on the real prompt textarea (composer.autocomplete.default, so
it outranks the Constructor modal it lives inside), and a stateless
/api/prompt/expand that combines anchor + shortcuts + $variables + postfix
at the moment a run is queued -- scene.text itself stays exactly what was
typed. Project gained anchor/postfix/variables editing UI on its own tab.
Along the way: a real layout bug in the Properties column (every composer
stack can flex-shrink below its own content, and once the Project tab grew
past a couple of rows the column started squeezing its own children's
content into each other and, at the outer level, into the sibling Generation
section below) -- fixed with flex-shrink:0 at each level that needs it,
caught by a Playwright regression test.
