feat(garmin): implement Authenticate/CompleteMFA on subprocessClient

Structured {status, message} responses replace the old string
pattern-matching (parseAuthResult) -- both sides of the protocol are now
owned by this repo, so there's no need to guess at phrasing anymore.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/internal/garmin/client_test.go

@@ -2,6 +2,7 @@ package garmin
 
 import (
 	"bufio"
+	"context"
 	"encoding/json"
 	"io"
 	"os/exec"
@@ -191,3 +192,87 @@ func TestSubprocessClient_Close_WaitsForStderrCopyGoroutine(t *testing.T) {
 		t.Error("stderrDone should be reset to nil after close()")
 	}
 }
+
+func TestSubprocessClient_Authenticate_Success(t *testing.T) {
+	c := newFakeWrapperClient(t, func(cmd string, params json.RawMessage) wireResponsePayload {
+		if cmd != "authenticate" {
+			t.Fatalf("unexpected cmd %q", cmd)
+		}
+		return fakeResult(authResultWire{Status: "success", Message: "Authenticated successfully."})
+	})
+
+	res, err := c.Authenticate(context.Background())
+	if err != nil {
+		t.Fatalf("Authenticate: %v", err)
+	}
+	if res.Status != AuthSuccess || res.Message != "Authenticated successfully." {
+		t.Errorf("Authenticate result = %+v", res)
+	}
+}
+
+func TestSubprocessClient_Authenticate_MFARequired(t *testing.T) {
+	c := newFakeWrapperClient(t, func(cmd string, params json.RawMessage) wireResponsePayload {
+		return fakeResult(authResultWire{Status: "mfa_required", Message: "MFA required. ..."})
+	})
+
+	res, err := c.Authenticate(context.Background())
+	if err != nil {
+		t.Fatalf("Authenticate: %v", err)
+	}
+	if res.Status != AuthMFARequired {
+		t.Errorf("Authenticate status = %v, want AuthMFARequired", res.Status)
+	}
+}
+
+func TestSubprocessClient_Authenticate_WrapperErrorPropagates(t *testing.T) {
+	c := newFakeWrapperClient(t, func(cmd string, params json.RawMessage) wireResponsePayload {
+		return fakeError("subprocess exploded")
+	})
+
+	_, err := c.Authenticate(context.Background())
+	if err == nil || !strings.Contains(err.Error(), "subprocess exploded") {
+		t.Fatalf("Authenticate error = %v, want it to mention the wrapper error", err)
+	}
+}
+
+func TestSubprocessClient_CompleteMFA_SendsCodeAndReturnsStatus(t *testing.T) {
+	var gotCode string
+	c := newFakeWrapperClient(t, func(cmd string, params json.RawMessage) wireResponsePayload {
+		if cmd != "complete_mfa" {
+			t.Fatalf("unexpected cmd %q", cmd)
+		}
+		var p struct {
+			Code string `json:"code"`
+		}
+		if err := json.Unmarshal(params, &p); err != nil {
+			t.Fatalf("unmarshal params: %v", err)
+		}
+		gotCode = p.Code
+		return fakeResult(authResultWire{Status: "success", Message: "MFA accepted. Authenticated successfully."})
+	})
+
+	res, err := c.CompleteMFA(context.Background(), "123456")
+	if err != nil {
+		t.Fatalf("CompleteMFA: %v", err)
+	}
+	if gotCode != "123456" {
+		t.Errorf("code sent to wrapper = %q, want 123456", gotCode)
+	}
+	if res.Status != AuthSuccess {
+		t.Errorf("CompleteMFA status = %v, want AuthSuccess", res.Status)
+	}
+}
+
+func TestSubprocessClient_CompleteMFA_Failed(t *testing.T) {
+	c := newFakeWrapperClient(t, func(cmd string, params json.RawMessage) wireResponsePayload {
+		return fakeResult(authResultWire{Status: "failed", Message: "Authentication failed after MFA: bad code"})
+	})
+
+	res, err := c.CompleteMFA(context.Background(), "000000")
+	if err != nil {
+		t.Fatalf("CompleteMFA: %v", err)
+	}
+	if res.Status != AuthFailed {
+		t.Errorf("CompleteMFA status = %v, want AuthFailed", res.Status)
+	}
+}