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/models/registry_test.go

@@ -0,0 +1,60 @@
+package models
+
+import (
+	"testing"
+)
+
+func TestModelRegistry_FindModel_Exact(t *testing.T) {
+	r := &ModelRegistry{
+		Providers: map[string]ProviderInfo{
+			"openai": {
+				Models: map[string]ModelMetadata{
+					"gpt-4o": {ID: "gpt-4o", Name: "GPT-4o"},
+				},
+			},
+		},
+	}
+	m := r.FindModel("gpt-4o")
+	if m == nil {
+		t.Fatal("expected to find gpt-4o")
+	}
+	if m.Name != "GPT-4o" {
+		t.Fatalf("expected GPT-4o, got %s", m.Name)
+	}
+}
+
+func TestModelRegistry_FindModel_Fuzzy(t *testing.T) {
+	r := &ModelRegistry{
+		Providers: map[string]ProviderInfo{
+			"openai": {
+				Models: map[string]ModelMetadata{
+					"gpt-4o": {ID: "gpt-4o", Name: "GPT-4o"},
+				},
+			},
+		},
+	}
+	// Fuzzy: "gpt-4o-2024-05-13" should match "gpt-4o"
+	m := r.FindModel("gpt-4o-2024-05-13")
+	if m == nil {
+		t.Fatal("expected fuzzy match")
+	}
+	if m.Name != "GPT-4o" {
+		t.Fatalf("expected GPT-4o, got %s", m.Name)
+	}
+}
+
+func TestModelRegistry_FindModel_NotFound(t *testing.T) {
+	r := &ModelRegistry{
+		Providers: map[string]ProviderInfo{
+			"openai": {
+				Models: map[string]ModelMetadata{
+					"gpt-4o": {ID: "gpt-4o", Name: "GPT-4o"},
+				},
+			},
+		},
+	}
+	m := r.FindModel("nonexistent-model")
+	if m != nil {
+		t.Fatal("expected nil for nonexistent model")
+	}
+}