feat: support updating Garmin credentials at runtime

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

backend/internal/garmin/client_test.go

@@ -20,3 +20,20 @@ func TestParseAuthResult(t *testing.T) {
 		}
 	}
 }
+
+func TestMcpClient_UpdateCredentials_ResetsStartedState(t *testing.T) {
+	c := &mcpClient{cfg: Config{GarminEmail: "old@example.com", GarminPassword: "old"}}
+	c.started = true // simulate an already-spawned subprocess
+
+	c.UpdateCredentials("new@example.com", "new")
+
+	if c.cfg.GarminEmail != "new@example.com" || c.cfg.GarminPassword != "new" {
+		t.Errorf("cfg after update = %+v, want new@example.com/new", c.cfg)
+	}
+	if c.started {
+		t.Error("started should be reset to false so the next call respawns the subprocess")
+	}
+	if c.inner != nil {
+		t.Error("inner should be cleared so ensureStarted spawns a fresh client")
+	}
+}