Backend was already V1-complete for these paths (moveFlexibleToBacklog,
breakUpTask); this closes the UI-wiring gap identified in the V1
feature-complete audit.
- Add createBacklogController() to SchedulerAppComposition and implement it
in PersistentSchedulerComposition (only the demo composition had one).
- Sidebar now switches the main content area between Today and Backlog via
a real SidebarScreen enum instead of hardcoded onTap: () {} stubs.
- Task modal's "Backlog" quick-action reuses the existing onPushToBacklog
wiring (same backend command as the Push menu's "Push to backlog").
- Add a break-up dialog (row-level title/priority/reward/duration per
MVP-AC-11) and SchedulerCommandController.breakUpTask(), wired to the
modal's "Break up" button.
- Fix a latent bug where the active sidebar nav item's key was hardcoded
to nav-today-active regardless of which item was active.
Closes: #9
Closes: #12
Closes: #13
