feat: add GeoIP country lookup with embedded DB-IP Lite database (PLAN.md 4.3)

Embeds a DB-IP Lite country MMDB (~5MB) in the binary via go:embed,
keeping the single-binary deployment story clean. Country codes are
stored alongside login attempts and sessions, shown in the dashboard
(Top IPs, Top Countries card, Recent/Active Sessions, session detail).

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

internal/shell/banking/banking_test.go

@@ -17,7 +17,7 @@ import (
 func newTestModel(t *testing.T) (*model, *storage.MemoryStore) {
 	t.Helper()
 	store := storage.NewMemoryStore()
-	sessID, _ := store.CreateSession(context.Background(), "127.0.0.1", "banker", "banking")
+	sessID, _ := store.CreateSession(context.Background(), "127.0.0.1", "banker", "banking", "")
 	sess := &shell.SessionContext{
 		SessionID: sessID,
 		Username:  "banker",