feat(difficulty): add difficulty-tier game mode with seed catalogs and home UI

Adds DifficultyLevel (Easy/Medium/Hard/Expert/Grandmaster/Random) to
solitaire_core::game_state alongside GameMode::Difficulty(DifficultyLevel).
Five seed catalogs (40 seeds each) are pre-verified by the new
gen_difficulty_seeds binary using tiered solver budgets (1K–200K moves).
DifficultyPlugin resolves StartDifficultyRequestEvent → catalog seed →
NewGameRequestEvent; Random uses a system-time seed and bypasses the
winnable-only filter. The home overlay gets an expandable Difficulty section
between Draw Mode and the mode grid; last-played tier persists in Settings.
Difficulty wins pool into Classic stats. 5 unit tests in difficulty_plugin.

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

Cargo.lock

@@ -6988,6 +6988,7 @@ dependencies = [
  "axum",
  "chrono",
  "dirs",
+ "jni 0.21.1",
  "jsonwebtoken",
  "keyring-core",
  "reqwest",
@@ -7011,6 +7012,7 @@ dependencies = [
  "bevy",
  "chrono",
  "dirs",
+ "jni 0.21.1",
  "kira",
  "resvg",
  "ron",