IsDeselectAllActive was identical to IsDeselectAll (both return the
deselectAll flag under a read lock). The two were introduced in parallel
on separate branches for the same fix - guarding against management route
sync overriding a user's explicit deselect-all - and ended up coexisting
after the branches merged.
Drop IsDeselectAllActive, keep the canonical IsDeselectAll from main, and
move its richer doc comment over. Update callers accordingly.
Exit nodes are mutually exclusive, but the RouteSelector stores routes with
default-on semantics, so every available exit node reported as selected at once.
Reconcile exit-node selection on each network map (and on runtime selection):
keep at most one selected — the user's persisted pick, else whatever management
marks for auto-apply (SkipAutoApply=false), else none. Never auto-activate an
exit node the map doesn't request; it stays off until the user picks it.
The server deselects sibling exit nodes when the user activates one (leaving
non-exit routes untouched), and the tray/React exit-node toggle now appends so
activating an exit node no longer wipes network-route selections.
