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/AbstractUITask.h

@@ -44,6 +44,9 @@ public:
   // An end-to-end ACK (CRC) arrived for one of our sent messages — drives the
   // DM delivery-status marker. Default no-op for UIs that don't track it.
   virtual void onMsgAck(uint32_t ack_crc) { (void)ack_crc; }
+  // A repeater rebroadcast of one of our channel sends was heard (seq from
+  // lastChannelRelaySeq()) — drives the channel "relayed into mesh" marker.
+  virtual void onChannelRelayed(uint32_t seq) { (void)seq; }
   // True only when a BLE central is actually bonded/connected. On a dual
   // (BLE+USB) interface hasConnection() is always true (USB counts), so use
   // this for BLE-specific UI like the pairing-PIN prompt.