feat(ui): channel "relayed into mesh" marker (✓ on repeater echo)

Channels have no recipient ACK, so true delivery can't be shown. Instead reuse
the heard-repeater-echo idea (same as APC's flood feedback) to mark an outgoing
channel message ✓ once a repeater rebroadcast of it is heard — i.e. it was
relayed into the mesh. No echo is NOT a failure (direct/0-hop neighbours never
echo), so unconfirmed sends simply show no marker.

- MyMesh: single-slot relay tracker independent of APC (trackRelaySend on every
  channel flood; hash-match in filterRecvFloodPacket gated on _relay_pending so
  the hot recv path is untouched otherwise; window expiry in loop just drops it).
  lastChannelRelaySeq() hands the seq to the UI.
- AbstractUITask::onChannelRelayed() -> UITask -> QuickMsgScreen::markChannelRelayed().
- ChHistEntry gains relay_status / relay_seq; afterSend arms the marker on the
  exact stored entry; render shows ✓ only on ACK_OK for our own ("Me:") rows.

Verified: GAT562 (SSD1306) and WioTrackerL1Eink (GxEPD) solo builds compile clean.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

examples/companion_radio/ui-new/UITask.cpp

@@ -1376,6 +1376,10 @@ void UITask::onMsgAck(uint32_t ack_crc) {
   ((QuickMsgScreen*)quick_msg)->markDmDelivered(ack_crc);
 }
 
+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);
 }