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/ui-new/UITask.cpp

@@ -1380,8 +1380,8 @@ void UITask::onChannelRelayed(uint32_t seq) {
   ((QuickMsgScreen*)quick_msg)->markChannelRelayed(seq);
 }
 
-void UITask::addDMMsg(const uint8_t* pub_key, bool outgoing, const char* text) {
-  ((QuickMsgScreen*)quick_msg)->addDMMsg(pub_key, outgoing, text);
+void UITask::addDMMsg(const uint8_t* pub_key, bool outgoing, const char* text, uint32_t sender_timestamp) {
+  ((QuickMsgScreen*)quick_msg)->addDMMsg(pub_key, outgoing, text, sender_timestamp);
 }
 
 int UITask::getDMUnreadTotal() const {
@@ -1610,6 +1610,9 @@ static void formatDashVal(uint8_t field, char* val, int val_len, uint16_t batt_m
 
 void UITask::loop() {
   char c = 0;
+  // Background delivery: resend pending on-device DMs whose ACK timed out, and
+  // finalise the ✗ marker — runs regardless of which screen is active.
+  ((QuickMsgScreen*)quick_msg)->tickDmResends();
 #if UI_HAS_JOYSTICK
   uint8_t joy_rot = _node_prefs ? _node_prefs->joystick_rotation : JOYSTICK_ROTATION;
   int ev = user_btn.check();