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/DataStore.cpp

@@ -324,6 +324,7 @@ void DataStore::loadPrefsInt(const char *filename, NodePrefs& _prefs, double& no
   rd(&_prefs.advert_sound_scope,  sizeof(_prefs.advert_sound_scope));
   rd(&_prefs.rx_powersave,        sizeof(_prefs.rx_powersave));
   rd(&_prefs.tx_apc,              sizeof(_prefs.tx_apc));
+  rd(&_prefs.dm_resend_count,     sizeof(_prefs.dm_resend_count));
   // These fields were appended over successive schema bumps; an older file
   // can leave stray bytes here, so clamp out-of-range values back to defaults.
   // Values for notif_melody_ad: 0=built-in, 1=melody1, 2=melody2, 3=none.
@@ -333,6 +334,9 @@ void DataStore::loadPrefsInt(const char *filename, NodePrefs& _prefs, double& no
   if (_prefs.advert_sound_scope > 1) _prefs.advert_sound_scope = ADVERT_SOUND_SCOPE_ALL;
   if (_prefs.rx_powersave    > 1) _prefs.rx_powersave    = 0;
   if (_prefs.tx_apc          > 1) _prefs.tx_apc          = 0;
+  // An old (0xC0DE0009) file leaves the low byte of its sentinel here (0x09),
+  // which is out of range — fall back to the default of 2 resends.
+  if (_prefs.dm_resend_count > 5) _prefs.dm_resend_count = 2;
 
   // Schema sentinel: bumped on layout changes. Mismatch means an older file
   // (or a different schema); rd() already zero-inits any fields not present,
@@ -456,6 +460,7 @@ void DataStore::savePrefs(const NodePrefs& _prefs, double node_lat, double node_
     file.write((uint8_t *)&_prefs.advert_sound_scope,  sizeof(_prefs.advert_sound_scope));
     file.write((uint8_t *)&_prefs.rx_powersave,        sizeof(_prefs.rx_powersave));
     file.write((uint8_t *)&_prefs.tx_apc,              sizeof(_prefs.tx_apc));
+    file.write((uint8_t *)&_prefs.dm_resend_count,     sizeof(_prefs.dm_resend_count));
 
     // Tail sentinel — must be last. See NodePrefs::SCHEMA_SENTINEL.
     uint32_t sentinel = NodePrefs::SCHEMA_SENTINEL;