feat: Phase 3 - architecture & maintainability

- Split 1474-line dashboard.go into 5 domain files (clients, providers, users, system)
- Unit tests for ModelRegistry.FindModel and CalculateCost
- go mod tidy + verify (deps clean)
- .gitignore excludes tool cache dirs (.pi-lens/, .opencode/)

internal/utils/registry_test.go

@@ -0,0 +1,40 @@
+package utils
+
+import (
+	"testing"
+
+	"gophergate/internal/models"
+)
+
+func TestCalculateCost_NotFound(t *testing.T) {
+	r := &models.ModelRegistry{Providers: make(map[string]models.ProviderInfo)}
+	cost := CalculateCost(r, "unknown-model", 100, 50, 0, 0, 0)
+	if cost != 0.0 {
+		t.Fatalf("expected 0 cost for unknown model, got %f", cost)
+	}
+}
+
+func TestCalculateCost_KnownModel(t *testing.T) {
+	inputCost := 2.5   // $2.50 per 1M tokens
+	outputCost := 10.0 // $10.00 per 1M tokens
+	r := &models.ModelRegistry{
+		Providers: map[string]models.ProviderInfo{
+			"openai": {
+				Models: map[string]models.ModelMetadata{
+					"gpt-4o": {
+						Cost: &models.ModelCost{
+							Input:  inputCost,
+							Output: outputCost,
+						},
+					},
+				},
+			},
+		},
+	}
+
+	cost := CalculateCost(r, "gpt-4o", 1000, 500, 0, 0, 0)
+	expected := (1000 * inputCost / 1000000.0) + (500 * outputCost / 1000000.0)
+	if cost != expected {
+		t.Fatalf("expected %f, got %f", expected, cost)
+	}
+}