test(data): client-side sync round-trip integration tests

Server-side endpoint tests already exist in solitaire_server. This
adds the client-side counterpart: five integration tests in
solitaire_data/tests/sync_round_trip.rs that drive
SolitaireServerClient against an in-process axum::serve harness with
an in-memory SQLite database, covering:

- register_login_push_pull_round_trip — happy path: register, push
  non-default stats, pull from a fresh client, assert the merged
  payload reflects the pushed values
- pull_after_concurrent_pushes_merges_correctly — two clients on one
  user push different games_played values, verify the server-side
  merge returns the max
- unauthenticated_pull_returns_authentication_error — pull without
  tokens surfaces SyncError::Auth as expected
- jwt_refresh_on_401_succeeds — replace the access token with one
  whose exp is two hours stale (same signing key), pull triggers
  401 → /api/auth/refresh → retry, asserts the call ultimately
  succeeds
- pull_after_account_deletion_returns_default_or_error — register,
  push, delete via the trait, confirm the next push surfaces a
  result rather than panicking

keyring_core's mock store is installed once per process via Once;
each test uses a unique username so the shared store doesn't
cross-contaminate. Production code in sync_client.rs needed no
changes — the Box<dyn SyncProvider> design plus the mock keyring
were sufficient to drive every flow from outside.

solitaire_server is added as a path dev-dependency along with the
direct crates the harness needs (axum, sqlx, jsonwebtoken, uuid,
chrono, solitaire_sync); no runtime deps changed.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Cargo.lock

@@ -7533,16 +7533,21 @@ name = "solitaire_data"
 version = "0.1.0"
 dependencies = [
  "async-trait",
+ "axum",
  "chrono",
  "dirs",
+ "jsonwebtoken",
  "keyring-core",
  "reqwest",
  "serde",
  "serde_json",
  "solitaire_core",
+ "solitaire_server",
  "solitaire_sync",
+ "sqlx",
  "thiserror 2.0.18",
  "tokio",
+ "uuid",
 ]
 
 [[package]]