fix(server): use char count (not byte length) for display_name limit

The leaderboard opt-in handler was calling `.len()` on the display name,
which returns byte count. Multi-byte Unicode characters (emoji, CJK, etc.)
would be rejected well before the 32-character visual limit and with a
misleading error message. Switched to `.chars().count()` to enforce the
limit in terms of Unicode scalar values as the error message advertises.

test(core): add boundary tests for 7 uncovered achievement conditions
test(server): add display_name validation integration tests (empty,
too-long ASCII, 32-emoji succeeds, 33-emoji rejected)

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

solitaire_server/src/leaderboard.rs

@@ -119,7 +119,7 @@ pub async fn opt_in(
     if display_name.is_empty() {
         return Err(AppError::BadRequest("display_name must not be empty".into()));
     }
-    if display_name.len() > DISPLAY_NAME_MAX {
+    if display_name.chars().count() > DISPLAY_NAME_MAX {
         return Err(AppError::BadRequest(format!(
             "display_name must be at most {DISPLAY_NAME_MAX} characters"
         )));