feat(garmin): expose ErrNotFound for a wrapper-reported 404

roundTrip now wraps the returned error with the new ErrNotFound
sentinel whenever the wrapper's response set not_found (Task 1),
letting any Client method's caller distinguish a definitive 404 from
a transient failure via errors.Is, regardless of which garminconnect
method was called.

backend/internal/garmin/client_test.go

@@ -5,6 +5,7 @@ import (
 	"bytes"
 	"context"
 	"encoding/json"
+	"errors"
 	"io"
 	"log/slog"
 	"os/exec"
@@ -18,8 +19,9 @@ import (
 // wireResponsePayload is what a fake wrapper handler returns for one
 // request; the harness fills in the response ID.
 type wireResponsePayload struct {
-	result json.RawMessage
-	err    string
+	result   json.RawMessage
+	err      string
+	notFound bool
 }
 
 func fakeResult(v any) wireResponsePayload {
@@ -34,6 +36,10 @@ func fakeError(msg string) wireResponsePayload {
 	return wireResponsePayload{err: msg}
 }
 
+func fakeNotFoundError(msg string) wireResponsePayload {
+	return wireResponsePayload{err: msg, notFound: true}
+}
+
 // newFakeWrapperClient wires a subprocessClient to an in-process goroutine
 // that plays the Python wrapper's role, so protocol-level Go logic can be
 // tested without python3/garminconnect installed.
@@ -61,7 +67,7 @@ func newFakeWrapperClient(t *testing.T, handle func(cmd string, params json.RawM
 				continue
 			}
 			payload := handle(req.Cmd, req.Params)
-			resp := wireResponse{ID: req.ID, Result: payload.result, Error: payload.err}
+			resp := wireResponse{ID: req.ID, Result: payload.result, Error: payload.err, NotFound: payload.notFound}
 			if err := enc.Encode(resp); err != nil {
 				return
 			}
@@ -134,6 +140,31 @@ func TestSubprocessClient_RoundTrip_WrapperErrorPropagates(t *testing.T) {
 	}
 }
 
+func TestSubprocessClient_RoundTrip_NotFoundWrapsErrNotFound(t *testing.T) {
+	c := newFakeWrapperClient(t, func(cmd string, params json.RawMessage) wireResponsePayload {
+		return fakeNotFoundError("API Error 404")
+	})
+
+	_, err := c.roundTrip(context.Background(), "call", nil)
+	if !errors.Is(err, ErrNotFound) {
+		t.Fatalf("roundTrip error = %v, want errors.Is(err, ErrNotFound)", err)
+	}
+	if !strings.Contains(err.Error(), "API Error 404") {
+		t.Errorf("roundTrip error = %v, want it to still contain the original message", err)
+	}
+}
+
+func TestSubprocessClient_RoundTrip_OrdinaryErrorDoesNotWrapErrNotFound(t *testing.T) {
+	c := newFakeWrapperClient(t, func(cmd string, params json.RawMessage) wireResponsePayload {
+		return fakeError("boom")
+	})
+
+	_, err := c.roundTrip(context.Background(), "call", nil)
+	if errors.Is(err, ErrNotFound) {
+		t.Fatalf("roundTrip error = %v, want errors.Is(err, ErrNotFound) to be false", err)
+	}
+}
+
 func TestSubprocessClient_UpdateCredentials_ResetsStartedState(t *testing.T) {
 	c := &subprocessClient{cfg: Config{GarminEmail: "old@example.com", GarminPassword: "old"}}
 	c.started = true // simulate an already-spawned subprocess, no real cmd/pipes