test: add unit tests for config generation and form values

Cover the deploy-critical pure logic:

- GenerateConfigTOML: round-trip decode of values + a guard asserting the
  config port is always containerAppPort (host port must not leak)
- GenerateWireguardConfig: MTU emitted/commented, PROTO default UDP
- WriteConfigFile / WriteWireguardConfigFile: numeric validation rejects
  bad input and does not write a file
- FormStep.Values(): text/password/select resolution and select default
  fallback

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

internal/tui/form_test.go

@@ -0,0 +1,64 @@
+package tui
+
+import "testing"
+
+func TestFormStepValues(t *testing.T) {
+	f := NewFormStep("Test", []FormField{
+		{
+			Id:      "name",
+			Label:   "Name",
+			Default: "john",
+			Type:    FieldTypeText,
+		},
+		{
+			Id:      "secret",
+			Label:   "Secret",
+			Default: "hunter2",
+			Type:    FieldTypePassword,
+		},
+		{
+			Id:      "proto",
+			Label:   "Protocol",
+			Default: "TCP",
+			Type:    FieldTypeSelect,
+			Options: []string{"UDP", "TCP"},
+		},
+		{
+			Id:      "mode",
+			Label:   "Mode",
+			Type:    FieldTypeSelect,
+			Options: []string{"a", "b", "c"},
+		},
+	})
+
+	values := f.Values()
+
+	want := map[string]string{
+		"name":   "john", // text default
+		"secret": "hunter2",
+		"proto":  "TCP", // select default resolves to the matching option
+		"mode":   "a",   // select with no default falls back to first option
+	}
+
+	for id, exp := range want {
+		if got := values[id]; got != exp {
+			t.Errorf("Values()[%q] = %q, want %q", id, got, exp)
+		}
+	}
+}
+
+func TestFormStepValues_SelectInvalidDefaultFallsBackToFirst(t *testing.T) {
+	f := NewFormStep("Test", []FormField{
+		{
+			Id:      "proto",
+			Label:   "Protocol",
+			Default: "NOPE", // not among options
+			Type:    FieldTypeSelect,
+			Options: []string{"UDP", "TCP"},
+		},
+	})
+
+	if got := f.Values()["proto"]; got != "UDP" {
+		t.Errorf("Values()[proto] = %q, want first option %q", got, "UDP")
+	}
+}