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.h

@@ -21,6 +21,9 @@ struct AddressBookEntry {
     // (shown only when that wallet is the active one). Empty is treated as "global" so legacy
     // entries — written before multi-wallet scoping — keep showing everywhere.
     std::string scope = "global";
+    // Contact avatar: "" = default type badge (Z/T), "icon:<name>" = a Material wallet-icon,
+    // "img:<path>" = a custom image (copied into <config>/contact-avatars/).
+    std::string avatar;
 
     AddressBookEntry() = default;
     AddressBookEntry(const std::string& l, const std::string& a, const std::string& n = "",