Add user profile management: display name + avatar upload (Gitea issue #12)

Users can set a display name (shown instead of username in the message
list, room member list, TopBar, and admin Users tab) and upload a real
avatar, replacing the generated color-initial avatars everywhere a user
appears. Avatars are square-cropped and downscaled to 512px, reusing
app/storage.py's upload primitives from image uploads with a new square
option.

Two deliberate divergences from message-image handling, documented in
backend/README.md: the previous avatar file is deleted on replace/remove
(safe since it's strictly one file per user), and avatar serving is not
room-gated and uses a short cache (identity-addressed and mutable, unlike
a message image's permanent content-addressed URL).

Frontend: new ProfileModal reachable from the TopBar account menu;
AuthContext gains updateUser() so a profile change reflects instantly
everywhere without a refetch.

frontend/src/components/Modal.css

@@ -182,3 +182,21 @@
   font-size: 0.76rem;
   color: var(--ds-muted);
 }
+
+.profile-modal-avatar-row {
+  display: flex;
+  align-items: center;
+  gap: var(--sp-4);
+  margin-bottom: var(--sp-4);
+}
+
+.profile-modal-avatar-actions {
+  display: flex;
+  flex-direction: column;
+  gap: var(--sp-2);
+  align-items: flex-start;
+}
+
+.modal-hidden-file-input {
+  display: none;
+}