fix(server): auth-guard avatar serving, atomic write, user_id assertion in merge

- Move /avatars ServeDir behind require_auth middleware so avatar files
  can only be fetched by authenticated users (H-11)
- Make avatar upload atomic via .tmp write + rename, cleaning up stale
  extensions only after the rename succeeds (H-12)
- Return 401 instead of silently returning an empty username string when
  the user row is unexpectedly missing a username (L-17)
- Add user_id mismatch guard to merge(): returns local payload unchanged
  with a ConflictReport rather than silently cross-contaminating data (H-2)
- Truncate opt-in display_name to 32 chars client-side before sending,
  matching the server's DISPLAY_NAME_MAX validation (L-5)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

solitaire_engine/src/leaderboard_plugin.rs

@@ -350,7 +350,7 @@ fn handle_opt_in_button(
                             None
                         }
                     })
-                    .map(str::to_string)
+                    .map(|n| n.chars().take(32).collect::<String>())
             })
             .unwrap_or_else(|| "Player".to_string());
 

solitaire_server/src/auth.rs

@@ -336,7 +336,7 @@ pub async fn get_me(
 
     Ok(Json(MeResponse {
         id: user.user_id,
-        username: row.username.unwrap_or_default(),
+        username: row.username.ok_or(AppError::Unauthorized)?,
         avatar_url: row.avatar_url,
     }))
 }
@@ -386,13 +386,19 @@ pub async fn upload_avatar(
     std::fs::create_dir_all("avatars").map_err(|e| AppError::Internal(e.to_string()))?;
     let filename = format!("{}.{}", user.user_id, ext);
     let path = std::path::Path::new("avatars").join(&filename);
-    // Remove stale files with other extensions first.
+    let tmp_path = std::path::Path::new("avatars").join(format!("{}.{}.tmp", user.user_id, ext));
+    // Write to a temp file then atomically rename so concurrent readers never
+    // see a partially-written avatar.
+    std::fs::write(&tmp_path, &body).map_err(|e| AppError::Internal(e.to_string()))?;
+    std::fs::rename(&tmp_path, &path).map_err(|e| AppError::Internal(e.to_string()))?;
+    // Remove stale files with other extensions after the atomic rename.
     for old_ext in &["jpg", "png", "webp", "gif"] {
-        let _ = std::fs::remove_file(
-            std::path::Path::new("avatars").join(format!("{}.{}", user.user_id, old_ext)),
-        );
+        if *old_ext != ext {
+            let _ = std::fs::remove_file(
+                std::path::Path::new("avatars").join(format!("{}.{}", user.user_id, old_ext)),
+            );
+        }
     }
-    std::fs::write(&path, &body).map_err(|e| AppError::Internal(e.to_string()))?;
 
     let avatar_url = format!("/avatars/{filename}");
     sqlx::query!(
@@ -412,7 +418,7 @@ pub async fn upload_avatar(
 
     Ok(Json(MeResponse {
         id: user.user_id,
-        username: username.unwrap_or_default(),
+        username: username.ok_or(AppError::Unauthorized)?,
         avatar_url: Some(avatar_url),
     }))
 }

solitaire_server/src/lib.rs

@@ -146,6 +146,7 @@ fn build_router_inner(state: AppState, rate_limit: bool) -> Router {
         .route("/api/account", delete(auth::delete_account))
         .route("/api/me", get(auth::get_me))
         .route("/api/me/avatar", put(auth::upload_avatar))
+        .nest_service("/avatars", ServeDir::new("avatars"))
         .layer(axum_middleware::from_fn_with_state(
             state.clone(),
             middleware::require_auth,
@@ -231,7 +232,6 @@ fn build_router_inner(state: AppState, rate_limit: bool) -> Router {
         )
         .nest_service("/web", ServeDir::new("solitaire_server/web"))
         .nest_service("/assets", ServeDir::new("assets"))
-        .nest_service("/avatars", ServeDir::new("avatars"))
         .layer(axum_middleware::from_fn(security_headers));
 
     Router::new()

solitaire_sync/src/merge.rs

@@ -47,6 +47,15 @@ use crate::progress::{level_for_xp, DAILY_CHALLENGE_HISTORY_CAP};
 pub fn merge(local: &SyncPayload, remote: &SyncPayload) -> (SyncPayload, Vec<ConflictReport>) {
     let mut conflicts = Vec::new();
 
+    if local.user_id != remote.user_id {
+        conflicts.push(ConflictReport {
+            field: "user_id".to_string(),
+            local_value: local.user_id.to_string(),
+            remote_value: remote.user_id.to_string(),
+        });
+        return (local.clone(), conflicts);
+    }
+
     let stats = merge_stats(&local.stats, &remote.stats, &mut conflicts);
     let achievements = merge_achievements(&local.achievements, &remote.achievements);
     let progress = merge_progress(&local.progress, &remote.progress, &mut conflicts);