fix(companion): prefs schema corruption, broken multi-scope, stale relay filter, cursor row

Follow-up review of f589b9b2 -- five defects in that commit's own changes.

- repeat_scope_only + repeat_extra_scopes were read/written in the MIDDLE of
  the prefs stream, beside their repeat_* siblings. loadPrefsInt()'s rd() is a
  plain sequential reader gated only on file.available(), with no per-field
  versioning, so on any pre-existing file those 25 bytes were taken from the
  fields that follow, shifting EVERY later field: repeater profile (incl. a
  float freq), track_shared_loc, all of loc_share_*, trail, bot, GPIO modes.
  Moved to the struct/file tail, sentinel bumped to 0xC0DE0027 with 0xC0DE0026
  marked burned. sizeof stays 2752 (confirmed by build); the tripwire procedure
  now spells out the append-only rule that "in struct order" left implicit.
- rebuildRepeatScopes() called getAutoKeyFor() with id 0 for every entry, but
  that cache is keyed on the id alone and ignores the name on a hit -- so every
  extra scope after the first silently got the first one's key, making the
  comma-separated list do nothing. Distinct id per scope now.
- interference_threshold had no load clamp, so an upgrader read 0x23 (35) out
  of the old file's sentinel tail instead of 0.
- CMD_SET_DEFAULT_FLOOD_SCOPE wrote default_scope_key without rebuilding the
  relay filter, so setting or clearing the scope from the app left the repeater
  filtering on the previous key until reboot. The on-device path already did.
- The keyboard preview derived the cursor's row a second time from byte
  offsets, disagreeing with the cursor_line the scroll window already computes:
  it pinned the cursor to the end of a full line (drawing '_' one character
  past the display width) at every wrap boundary. Use cursor_line directly.

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

examples/companion_radio/MyMesh.cpp

@@ -691,7 +691,11 @@ void MyMesh::rebuildRepeatScopes() {
     if (*tok != '\0') {
       char hashtag[1 + sizeof(_prefs.repeat_extra_scopes)];
       snprintf(hashtag, sizeof(hashtag), "#%s", tok);
-      temp.getAutoKeyFor(0, hashtag, repeat_scopes[repeat_scope_count++]);
+      // Distinct id per scope: getAutoKeyFor() keys its cache on the id ALONE
+      // and ignores the name on a hit, so reusing one id here would hand every
+      // scope after the first the first one's key.
+      temp.getAutoKeyFor(repeat_scope_count, hashtag, repeat_scopes[repeat_scope_count]);
+      repeat_scope_count++;
     }
     tok = strtok(NULL, ",");
   }
@@ -2807,6 +2811,7 @@ void MyMesh::handleCmdFrame(size_t len) {
       if (n > 0 && n < 31) {
         strcpy(_prefs.default_scope_name, (char *) &cmd_frame[1]);
         memcpy(_prefs.default_scope_key, &cmd_frame[1+31], 16);
+        rebuildRepeatScopes();   // slot 0 of the relay filter tracks this key
         savePrefs();
         writeOKFrame();
       } else {
@@ -2815,6 +2820,7 @@ void MyMesh::handleCmdFrame(size_t len) {
     } else {
       memset(_prefs.default_scope_name, 0, sizeof(_prefs.default_scope_name));  // set default scope to null
       memset(_prefs.default_scope_key, 0, sizeof(_prefs.default_scope_key));
+      rebuildRepeatScopes();   // drop it from the relay filter too, not just from sends
       savePrefs();
       writeOKFrame();
     }