Show real photos for people instead of scene art

Your avatar is your photo, picked from the device by tapping it on Home or
from a new Profile photo row in Settings, shrunk to 256 px and kept on the
phone (there is no upload endpoint yet). Group members show the photo the
server sends in a new `photo` field, everywhere avatars appear. Without a
photo, an initial on a neutral disc. Mock mode bundles free Unsplash
portraits for the sample people.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KhQh614DMeYoQEH9SoMRUw

frontend/assets/images/PEOPLE-CREDITS.txt

@@ -0,0 +1,13 @@
+Sample portraits for mock mode (person_*.jpg), from Unsplash under the
+Unsplash License (free to use, no attribution required). Source photo ids:
+
+person_alex.jpg    photo-1507003211169-0a1dd7228f2d
+person_sam.jpg     photo-1500648767791-00dcc994a43e
+person_priya.jpg   photo-1494790108377-be9c29b29330
+person_jordan.jpg  photo-1472099645785-5658abf4ff4e
+person_mei.jpg     photo-1544005313-94ddf0286df2
+person_tom.jpg     photo-1506794778202-cad84cf45f1d
+person_ade.jpg     photo-1531123897727-8f129e1688ce
+person_lena.jpg    photo-1438761681033-6461ffad8d80
+
+They stand in for real users' photos in the preview only.