feat(profile): default avatars via DiceBear "thumbs" (CC0), drop boring_avatars

Replace the flutter_boring_avatars fallback with a friendly DiceBear
"thumbs" face, generated offline (dicebear_core) and deterministically
from the pubkey, rendered as SVG (flutter_svg). The thumbs style is
CC0 1.0 (public domain) — no attribution, AGPL-clean — unlike Multiavatar
whose custom licence restricts use. Style parsed once and per-pubkey SVG
memoized; the CC0 <metadata> block is stripped to keep flutter_svg quiet.

pubspec.lock

@@ -297,6 +297,30 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "0.7.14"
+  dicebear_core:
+    dependency: transitive
+    description:
+      name: dicebear_core
+      sha256: "7a38725ed8895feeeb6d34a6a0946322a03649077fa49f9a711530e035fb53f2"
+      url: "https://pub.dev"
+    source: hosted
+    version: "10.3.0"
+  dicebear_schema:
+    dependency: transitive
+    description:
+      name: dicebear_schema
+      sha256: dfbaa477e4e7ed1500ae8f0263ecf7db3081c492489b817a401a52e21e0ace9c
+      url: "https://pub.dev"
+    source: hosted
+    version: "1.3.0"
+  dicebear_styles:
+    dependency: transitive
+    description:
+      name: dicebear_styles
+      sha256: "4761fc26da7eb12b5b499be4efbd7cadc9186e301095199bbf424a3150bcd866"
+      url: "https://pub.dev"
+    source: hosted
+    version: "10.2.0"
   drift:
     dependency: transitive
     description:
@@ -414,14 +438,6 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "9.1.1"
-  flutter_boring_avatars:
-    dependency: transitive
-    description:
-      name: flutter_boring_avatars
-      sha256: c4439a5e6753db58debd8d85a92e9747727e5f5c05cc1f3a252ba70d2c1df720
-      url: "https://pub.dev"
-    source: hosted
-    version: "2.1.0"
   flutter_driver:
     dependency: transitive
     description: flutter
@@ -536,6 +552,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "3.1.2"
+  flutter_svg:
+    dependency: transitive
+    description:
+      name: flutter_svg
+      sha256: "35882981abcbfb8c15b286f0cd690ff25bac12d95eff3e25ee207f37d4c42e7f"
+      url: "https://pub.dev"
+    source: hosted
+    version: "2.3.0"
   flutter_tesseract_ocr:
     dependency: transitive
     description:
@@ -812,6 +836,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "4.12.0"
+  json_schema:
+    dependency: transitive
+    description:
+      name: json_schema
+      sha256: f37d9c3fdfe8c9aae55fdfd5af815d24ce63c3a0f6a2c1f0982c30f43643fa1a
+      url: "https://pub.dev"
+    source: hosted
+    version: "5.2.2"
   leak_tracker:
     dependency: transitive
     description:
@@ -1124,6 +1156,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "3.0.2"
+  quiver:
+    dependency: transitive
+    description:
+      name: quiver
+      sha256: ea0b925899e64ecdfbf9c7becb60d5b50e706ade44a85b2363be2a22d88117d2
+      url: "https://pub.dev"
+    source: hosted
+    version: "3.2.2"
   recase:
     dependency: transitive
     description:
@@ -1140,6 +1180,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "0.6.0"
+  rfc_6901:
+    dependency: transitive
+    description:
+      name: rfc_6901
+      sha256: "6a43b1858dca2febaf93e15639aa6b0c49ccdfd7647775f15a499f872b018154"
+      url: "https://pub.dev"
+    source: hosted
+    version: "0.2.1"
   serial_csv:
     dependency: transitive
     description:
@@ -1361,6 +1409,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "2.3.1"
+  uri:
+    dependency: transitive
+    description:
+      name: uri
+      sha256: "889eea21e953187c6099802b7b4cf5219ba8f3518f604a1033064d45b1b8268a"
+      url: "https://pub.dev"
+    source: hosted
+    version: "1.0.0"
   url_launcher:
     dependency: transitive
     description:
@@ -1433,6 +1489,30 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "4.5.3"
+  vector_graphics:
+    dependency: transitive
+    description:
+      name: vector_graphics
+      sha256: "2306c03da2ba81724afeb589c351ebbc0aa7d86005925be8f8735856dbe5e42d"
+      url: "https://pub.dev"
+    source: hosted
+    version: "1.2.2"
+  vector_graphics_codec:
+    dependency: transitive
+    description:
+      name: vector_graphics_codec
+      sha256: "99fd9fbd34d9f9a32efd7b6a6aae14125d8237b10403b422a6a6dfeac2806146"
+      url: "https://pub.dev"
+    source: hosted
+    version: "1.1.13"
+  vector_graphics_compiler:
+    dependency: transitive
+    description:
+      name: vector_graphics_compiler
+      sha256: "142a9146f447d15b10bdc00e21d5f4d83e5b32bb5f8f8f5a04c75311344923a3"
+      url: "https://pub.dev"
+    source: hosted
+    version: "1.2.6"
   vector_math:
     dependency: transitive
     description: