feat(ui): Favourites phase 3 — in-place pin picker on empty tile

Enter on an empty Favourites slot opens a PopupMenu listing candidate
contacts. Sourced first from upstream-favourited chat contacts (those
flagged with c.flags & 0x01), then from recent DM contacts (resolved
from the 4-byte _dm_hist prefix to a full 6-byte pub_key via the contact
list), deduped against the favourites list. Capped at 12 entries.

Selection writes the chosen 6-byte prefix into the target slot; if the
contact is already pinned elsewhere, the previous slot is vacated first
(one contact per dial, per the agreed pin-only semantics). savePrefs
commits to flash and a brief alert confirms.

Plumbing:
- New QuickMsgScreen::getRecentDMContacts() resolves recent DM senders
  to 6-byte pub_key prefixes, deduped, newest first
- UITask::getRecentDMContacts() wraps it so HomeScreen doesn't reach
  into the QuickMsg screen directly
- HomeScreen gains its own PopupMenu + per-entry pub_key / label buffers
  (12 × 6 B keys + 12 × 22 B labels), no shared state with QuickMsg's
  Contact-options menu

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

examples/companion_radio/ui-new/QuickMsgScreen.h

@@ -475,6 +475,32 @@ public:
     _viewing_max_seen = 0;
   }
 
+  // Recent DM contacts, newest first, deduped. Resolves the 4-byte
+  // _dm_hist prefix to a 6-byte pub_key prefix by walking the contact
+  // list once per unique sender. Returns the number filled.
+  int getRecentDMContacts(uint8_t out[][NodePrefs::FAVOURITE_PREFIX_LEN], int max) const {
+    int n = 0;
+    for (int i = _dm_hist_count - 1; i >= 0 && n < max; i--) {
+      int pos = (_dm_hist_head + i) % DM_HIST_MAX;
+      const uint8_t* p4 = _dm_hist[pos].prefix;
+      // Skip if already collected.
+      bool dup = false;
+      for (int j = 0; j < n; j++) if (memcmp(out[j], p4, 4) == 0) { dup = true; break; }
+      if (dup) continue;
+      // Find a real contact whose pub_key starts with this 4-byte prefix.
+      for (int idx = 0; ; idx++) {
+        ContactInfo c;
+        if (!the_mesh.getContactByIdx(idx, c)) break;
+        if (memcmp(c.id.pub_key, p4, 4) == 0) {
+          memcpy(out[n], c.id.pub_key, NodePrefs::FAVOURITE_PREFIX_LEN);
+          n++;
+          break;
+        }
+      }
+    }
+    return n;
+  }
+
   // Jump straight into a contact's DM history (used by the Favourites dial).
   // Caller must have already reset() the screen. Marks the entry so KEY_CANCEL
   // from DM_HIST returns to the home screen instead of falling back through