feat(companion): on-device scope + repeater scope filtering; fix CAD, UTF-8 truncation, Public channel, Nodes list, keyboard cursor

- Settings > Radio > Scope: type a community/region name on-device (derives
  the shared key the same "#name" -> SHA256 way as DEFAULT_FLOOD_SCOPE_NAME),
  previously only settable from a connected app.
- Tools > Repeater > Scope only + Extra scopes: only relay flood traffic
  matching the device's own scope or a comma-separated list of additional
  scopes, without changing what scope the device's own messages send under.
  No-op while unconfigured.
- getCADEnabled()/getInterferenceThreshold() were hardcoded off on
  companion_radio; CAD now auto-enables whenever RX power-save (duty-cycle)
  is active, since the noise floor isn't kept fresh during duty-cycle sleep.
- Message truncation to fit the send frame could split a multi-byte UTF-8
  character in half; now stops at the last complete character.
- The default "Public" channel was unconditionally re-added at every boot
  before the saved channel list was loaded, so deleting it never stuck.
  Only seeded now on a genuinely fresh device (no channel file yet).
- Tools > Nodes read contacts from the wrong starting offset, landing on
  internally-reserved bookkeeping slots instead of real contacts -- showed
  as blank "Unknown" rows and silently dropped that many real contacts off
  the end of the list.
- resetContacts() only cleared the first few reserved slots, not the whole
  contact table, contrary to its own comment; only reachable today via
  private-key import, fixed to match stated intent regardless.
- Keyboard's multi-line text preview could render the cursor on an empty
  line below short typed text instead of right after it.

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

examples/companion_radio/ui-new/NearbyScreen.h

@@ -173,7 +173,16 @@ class NearbyScreen : public UIScreen {
     int nc = the_mesh.getNumContacts();
     for (int i = 0; i < nc && _count < MAX_NEARBY; i++) {
       ContactInfo ci;
-      if (!the_mesh.getContactByIdx(i, ci)) continue;
+      // getContactByIdx() indexes the RAW contact table, whose first
+      // MAX_ANON_CONTACTS slots are reserved for anon-request bookkeeping
+      // (see BaseChatMesh::resetContacts()/ContactsIterator) -- getNumContacts()
+      // already excludes them from the count, so real contact 0 lives at raw
+      // index MAX_ANON_CONTACTS, not 0. Reading from 0 pulled those reserved
+      // (blank, type=ADV_TYPE_NONE) slots into the list as bogus "Unknown"
+      // rows, and silently dropped the same number of real contacts off the
+      // end -- while never touching the anon slots themselves, so it always
+      // reproduced the same way regardless of the auto-add overwrite setting.
+      if (!the_mesh.getContactByIdx(i + MAX_ANON_CONTACTS, ci)) continue;
       if (!typeMatchesFilter(ci.type, ci.flags, true)) continue;
 
       Entry& e = _entries[_count++];
@@ -188,7 +197,7 @@ class NearbyScreen : public UIScreen {
                     ? geo::haversineKm(_own_lat, _own_lon, ci.gps_lat, ci.gps_lon)
                     : -1.0f;
       e.type        = ci.type;
-      e.contact_idx = i;
+      e.contact_idx = i + MAX_ANON_CONTACTS;   // raw index -- other lookups re-key off this directly
       e.lastmod     = ci.lastmod;
       e.is_known    = true;
       e.is_live     = false;