feat(companion): contact expiry + prune, and scope-list follow-up fixes

Settings > Contacts gains "Expire" (Off/7d/30d/90d) and a "Prune now" action
that counts first and asks before removing anything. A contact with no
advert/update within the threshold is eligible; favourites are always kept.
Thresholds and their labels live in one NodePrefs table so the age shown and
the age enforced can't drift. SCHEMA_SENTINEL -> 0xC0DE002C (sizeof unchanged
at 2824, confirmed on native and a real WioTrackerL1 build).

Also fixes four bugs in the scope list from 89c02eea:

- removeScope() saved /scopes1 but never savePrefs(), so the ch_scope_idx[]
  and repeat_extra_scope_mask fix-ups it makes were lost on reboot, leaving
  shifted entries against unshifted indices.
- CMD_SET_DEFAULT_FLOOD_SCOPE wrote the legacy fields directly instead of
  going through setPrimaryScope(), so the app's default-scope setting had
  nothing reading it once sends resolved through the list.
- The reverse direction was stale too: an on-device "Set as default" never
  refreshed default_scope_name/key, so CMD_GET_DEFAULT_FLOOD_SCOPE reported
  a scope the device had stopped using. New syncLegacyDefaultScope().
- Upgrading from the old single Scope field set the default (so DMs kept it)
  but left every channel unscoped, since "*" means unscoped, not "inherit".
  loadScopeList() now reports when it migrated and begin() seeds the channels
  that already exist, leaving empty slots alone.

"Set default" -> "Set as default", and setting it now says what it governs
("Default: DMs + relay") rather than leaving the [default] marker to imply
more than it does.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

examples/companion_radio/ui-new/UITask.cpp

@@ -3277,7 +3277,8 @@ void UITask::onContactRemoved(const uint8_t* pub_key) {
 // so a channel re-added at a freed slot can't inherit the old one's settings.
 // If you add such a field, add its cleanup below (and mark it in NodePrefs.h).
 // Currently covered: bot_channel_idx, loc_share_channel_idx, ch_notif_melody_*,
-// ch_notif_override/ch_notif_muted, ch_fav_bitmask, favourite_contacts/_kinds.
+// ch_notif_override/ch_notif_muted, ch_fav_bitmask, favourite_contacts/_kinds,
+// ch_scope_idx.
 void UITask::onChannelRemoved(uint8_t channel_idx) {
   if (!_node_prefs) return;
   bool changed = false;