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.go

@@ -8,6 +8,7 @@ import (
 	"context"
 	_ "embed"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"io"
 	"log/slog"
@@ -77,11 +78,20 @@ type wireRequest struct {
 
 // wireResponse is one line read from the wrapper subprocess's stdout.
 type wireResponse struct {
-	ID     int             `json:"id"`
-	Result json.RawMessage `json:"result,omitempty"`
-	Error  string          `json:"error,omitempty"`
+	ID       int             `json:"id"`
+	Result   json.RawMessage `json:"result,omitempty"`
+	Error    string          `json:"error,omitempty"`
+	NotFound bool            `json:"not_found,omitempty"`
 }
 
+// ErrNotFound wraps any error a Client method returns when the wrapper
+// reported a definitive HTTP 404 (garminconnect's own
+// GarminConnectNotFoundError) -- e.g. GetWorkoutByID for a workout deleted
+// on Garmin's side after being linked to an activity. Callers use
+// errors.Is(err, ErrNotFound) to distinguish this from a transient failure
+// worth retrying.
+var ErrNotFound = errors.New("garmin: resource not found")
+
 // callParams is the Params payload for a generic "call" request: dispatches
 // to any garminconnect.Garmin method by name.
 type callParams struct {
@@ -254,7 +264,11 @@ func (c *subprocessClient) roundTrip(ctx context.Context, cmdName string, params
 		return nil, err
 	}
 	if resp.Error != "" {
-		err = fmt.Errorf("%s: %s", cmdName, resp.Error)
+		if resp.NotFound {
+			err = fmt.Errorf("%s: %s: %w", cmdName, resp.Error, ErrNotFound)
+		} else {
+			err = fmt.Errorf("%s: %s", cmdName, resp.Error)
+		}
 		return nil, err
 	}
 	result = resp.Result