feat(ui): DM auto-resend + incoming dedup + channel relay ring

DM auto-resend (delivery status follow-up):
- New pref dm_resend_count (0-5, default 2), Settings › Messages › Resend.
  Schema sentinel 0xC0DE0009 → 0xC0DE000A; old files clamp to the default.
- When a pending on-device DM passes its ACK deadline, tickDmResends()
  (driven from UITask::loop, so it runs in the background regardless of the
  active screen) re-sends with the next attempt# reusing the original
  timestamp, refreshing ack_tag/deadline, until resends run out → then ✗.
- dmEffectiveStatus keeps the entry pending while resends remain.

Incoming DM dedup:
- A retry reuses the sender timestamp + text but has a fresh packet hash, so
  the mesh dup-filter passes it. addDMMsg now takes sender_timestamp and drops
  copies matching prefix+timestamp+text. sender_timestamp plumbed from
  MyMesh::queueMessage through AbstractUITask/UITask.

Channel relay ring:
- Replace the single-slot relayed-into-mesh tracker with a 4-slot ring so a
  quick burst of channel sends are each matched to their repeater echo.
  Receive-path hashing still gated (now on _relay_active) so the hot flood
  path is untouched when idle.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

examples/companion_radio/NodePrefs.h

@@ -131,11 +131,16 @@ struct NodePrefs {  // persisted to file
   // below the ceiling, so disabling restores the user's configured power.
   uint8_t  tx_apc;
 
+  // Auto-resend for on-device DMs: number of extra send attempts (0..5) made when
+  // no end-to-end ACK arrives before the deadline, before the delivery marker
+  // shows ✗. 0 = no auto-resend (single attempt). Default 2.
+  uint8_t  dm_resend_count;
+
   // Tail sentinel written at the end of /new_prefs. Bump the low byte when
   // adding/removing/reordering fields in DataStore::savePrefs/loadPrefsInt so
   // older saves are detected on load and skipped (zero-init defaults kept).
   // High 24 bits identify the file format; low byte is the schema revision.
-  static const uint32_t SCHEMA_SENTINEL = 0xC0DE0009;
+  static const uint32_t SCHEMA_SENTINEL = 0xC0DE000A;
 
   // Bit-index for each home page. Used by page_order (entries store bit+1) and
   // by home_pages_mask. Single source of truth — both HomeScreen::pageBit/bitToPage