Phase 20: achievements tab and unlock toasts

New Achievements tab (tab-achievements) shows all 18 achievement cards in
a responsive grid. Each card displays: icon, title, rarity badge (color-
coded: common=grey, rare=blue, epic=purple), description, reward coins, and
either a green "Unlocked <date>" label or a dimmed "Locked" state. Unlocked
achievements sort first (newest-first); locked sort alphabetically after.

Header nav button shows earned count badge next to "Achievements".

Match result handler reads achievements_unlocked from the Core response and
fires one toast per newly unlocked achievement immediately after match
submission. Level-up toasts continue to work independently.

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

tests/proxy_test.rs

@@ -251,6 +251,7 @@ async fn test_dashboard_contains_key_sections() {
     assert!(html.contains("tab-catalog"), "card catalog tab missing");
     assert!(html.contains("tab-settings"), "settings tab missing");
     assert!(html.contains("tab-notifications"), "notifications tab missing");
+    assert!(html.contains("tab-achievements"), "achievements tab missing");
 }
 
 #[tokio::test]