feat(ui): message delivery status (DM ACK + channel relay)

End-to-end delivery indicators on the device UI, drawn next to outgoing
messages and auto-scaled to the font (legible on landscape e-ink).

DM (and room servers, which share the DM path):
- Pending / delivered / failed marker driven by the real end-to-end ACK.
  Pending shows a row of dots — one per send — so auto-resend progress is
  visible before it resolves to ✓ / ✗.
- Auto-resend: new pref dm_resend_count (0-5, default 2), Settings ›
  Messages › Resend. A pending DM whose ACK times out is re-sent (reusing
  the original timestamp) until resends run out, then ✗. Driven from
  UITask::loop so it completes in the background, independent of screen.
- Incoming dedup: a retry reuses the sender timestamp + text but carries a
  fresh packet hash, so addDMMsg drops copies matching prefix+ts+text.

Channels (flood, no recipient ACK):
- ✓ only once a repeater echo confirms the send was relayed into the mesh;
  no echo is normal, not a failure (no pending/fail shown). A small ring
  tracks a burst of sends so each matches its echo. Receive-path hashing is
  gated so the hot flood path is untouched when idle.

Shared:
- Markers shown in both the history list and the fullscreen message view.
- Reusable scalable mini-icon facility in icons.h (bitmap + auto-scale);
  adding a new status icon is a bitmap plus one draw call.

No changes to the upstream mesh library (src/).

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

examples/companion_radio/ui-new/UITask.h

@@ -171,7 +171,9 @@ public:
   bool isMelodyPlaying();
   void showAlert(const char* text, int duration_millis);
   void addChannelMsg(uint8_t channel_idx, const char* text) override;
-  void addDMMsg(const uint8_t* pub_key, bool outgoing, const char* text) override;
+  void addDMMsg(const uint8_t* pub_key, bool outgoing, const char* text, uint32_t sender_timestamp = 0) override;
+  void onMsgAck(uint32_t ack_crc) override;
+  void onChannelRelayed(uint32_t seq) override;
   int  getDMUnreadTotal() const;
   int  getMsgCount() const { return _msgcount; }
   int  getChannelUnreadCount() const;