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/AbstractUITask.h

@@ -62,6 +62,6 @@ public:
   virtual void newMsg(uint8_t path_len, const char* from_name, const char* text, int msgcount, uint8_t contact_type = 0, const uint8_t* pub_key = nullptr) = 0;
   virtual void notify(UIEventType t = UIEventType::none) = 0;
   virtual void addChannelMsg(uint8_t channel_idx, const char* text) {}
-  virtual void addDMMsg(const uint8_t* pub_key, bool outgoing, const char* text) {}
+  virtual void addDMMsg(const uint8_t* pub_key, bool outgoing, const char* text, uint32_t sender_timestamp = 0) {}
   virtual void loop() = 0;
 };