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.

backend/app/schemas/user.py

@@ -18,4 +18,10 @@ class UserRead(BaseModel):
     email: EmailStr
     is_bot: bool
     is_site_admin: bool
+    display_name: str | None
+    avatar_filename: str | None
     created_at: datetime
+
+
+class ProfileUpdate(BaseModel):
+    display_name: str | None = Field(default=None, max_length=50)