Every Settings change now persists on its own (debounced ~500ms so a slider drag
is one PUT), with a small transient indicator; the draft/dirty-save UI and the
navigation leave-guard are gone. Also unblocks the navigation extraction — there
is no longer a cross-page "unsaved settings" guard to thread through routing.
Locks the five flows the refactor must not break (feed, channel, player,
settings, nav) so later sprints can prove they changed nothing. Adds a thin
data-testid layer and a dedicated seeded non-demo account; the suite runs on
the baked :8080 stack (authoritative) or :5173. No product-behaviour change.
peter