Filter the file list by the host's active project — flows without
a project: key count as 'general', display-only, the file is never
rewritten. New flows are stamped with the active project's key
(general and all-projects stay unstamped). Adds a toolbarTrailing
slot so the host can mount its workspace switcher in the editor's
single toolbar, and a flowsDir injection point so widget tests run
against a temp dir instead of the operator's ~/.chain flows.
Covered by pure filter-semantics tests plus hermetic widget tests
for filtering, the project-empty state, and new-flow stamping.
Signed-off-by: flemming-it <sf@flemming.it>
