feat(settings): rework settings page; remove unused SkeletonLoader/StorageManagement

Settings page refactor plus supporting docs (requirements, ux-flows,
traceability) and the frontend-domain-model spec with implementation-status
banner. Removes SkeletonLoader and StorageManagement components (no remaining
references).

docs/ux-flows.md

@@ -1076,6 +1076,13 @@ flowchart TB
 - **Mobile:** Click three-dot overflow menu → Select "Settings"
 - **Direct:** Navigate to `/settings` route
 
+**Settings apply instantly.** Every control on the Settings page persists the
+moment the user changes it — toggling a switch, picking a level, or releasing a
+slider writes that setting immediately. There is **no "Save" button** and no
+save/dirty state to reason about; leaving the page never risks losing a change.
+Sliders update their live readout while dragging but only persist on release
+(`change`, not each `input` tick) to avoid flooding the backend.
+
 ### 8.2 Logout Flow
 
 ```mermaid