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/design.md

@@ -51,6 +51,7 @@ directly above a large dark value. Used everywhere numbers appear.
 
 - Softly rounded, never pill-soft: large cards and image cards `16`, buttons `14`, rows, tiles and fields `12`, small controls `10`, chips and badges `8`, bars `4`. Sheets round their top at `20`.
 - Portraits, the nav buttons, status dots, rings and the breathing circle are full circles.
+- People are real photos (`designs/Portrait.dart`), never scene art: your photo is picked in Settings or by tapping the Home avatar, shrunk to 256 px and kept on the phone; members show the `photo` the server sends. No photo means an initial on a soft neutral disc. Mock mode uses bundled Unsplash portraits (`assets/images/person_*.jpg`, see `PEOPLE-CREDITS.txt`).
 - Screen padding `20` horizontal. Card padding `16`. Section gap `28`.
 - Hairlines at `colorDivider` (black 7%), borders at `colorBorder` (black 8%).
 - One soft shadow only: `0 4 18 rgba(0,0,0,0.03)`. No coloured glows.