feat(ui): DM delivery status marker (pending / delivered / failed)

Outgoing direct messages now show a small delivery glyph after "Me" in the DM
history, driven by MeshCore's existing end-to-end ACK (no protocol change):
- · pending (awaiting ACK)
- ✓ delivered (ACK matched)
- ✗ failed (no ACK by the deadline)

Plumbing:
- DmHistEntry gains ack_status / ack_tag / ack_deadline_ms (RAM only).
- sendText() captures expected_ack + est_timeout; afterSend() stores them.
- AbstractUITask::onMsgAck() virtual; MyMesh::onAckRecv routes ours-only ACKs
  to UITask -> QuickMsgScreen::markDmDelivered().
- Status evaluated lazily (pending past deadline reads as failed); no timer.
- Glyphs hand-drawn (no font has a usable check/cross), font-independent.

Sends with no path (expected_ack == 0) show no marker (can't be confirmed).

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

examples/companion_radio/AbstractUITask.h

@@ -41,6 +41,9 @@ public:
   }
   bool hasConnection() const { return _connected; }
   virtual void onBLEDisconnected() {}
+  // 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; }
   // 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.