feat(contacts): contact avatars — custom image / Material icon / Z-T badge

Add an `avatar` field to AddressBookEntry ("" = default Z/T type badge,
"icon:<name>" = a Material wallet-icon, "img:<path>" = a custom image),
serialized additively in addressbook.json (only written when non-empty, so
existing books are untouched).

Render it in the Cards/List views via a new drawContactAvatar helper: custom
images are loaded once through a path-keyed texture cache and drawn
circular-cropped (centre-cropped UVs + a thin border ring); icons reuse the
project-icon set (incl. the special pickaxe font path) in a tinted circle;
everything else falls back to the existing Z/T badge (also the fallback when
an image fails to load or an icon name is unknown).

Seed one sweep contact with an icon avatar to exercise the icon-badge path.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/data/address_book.cpp

@@ -54,6 +54,7 @@ bool AddressBook::load()
                 // Legacy entries (no "scope") migrate to "global" so nothing disappears when
                 // multi-wallet scoping lands — a contact you already had stays visible everywhere.
                 e.scope = entry.value("scope", "global");
+                e.avatar = entry.value("avatar", "");
 
                 if (!e.address.empty()) {
                     entries_.push_back(e);
@@ -86,6 +87,7 @@ bool AddressBook::save()
             e["address"] = entry.address;
             e["notes"] = entry.notes;
             e["scope"] = entry.scope.empty() ? std::string("global") : entry.scope;
+            if (!entry.avatar.empty()) e["avatar"] = entry.avatar;
             j["entries"].push_back(e);
         }