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/app_sweep.cpp

@@ -100,6 +100,7 @@ void seedSweepContacts(App& a)
 {
     s_contactsSweepBackup = a.addressBook().entries();
     data::AddressBookEntry e1("drgx pool payout address", kDemoZAddr, "mining pool payouts");   // Z, global
+    e1.avatar = "icon:account_balance";   // icon avatar (verifies the icon-badge render path)
     data::AddressBookEntry e2("exchange deposit", "t1DemoTransparentAddressForUiSweep00000", ""); // T
     // Scope to the active wallet so it's visible (the tab hides contacts not in the active wallet);
     // non-empty hash also keeps it non-global -> no globe badge, so the list shows a global/non-global mix.