fix(defter): haftalar ayın günlerine göre (1–8, 9–15, 16–22, 23–son), defter bugünün haftasıyla açılır

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

backend/tests/periods.rs

@@ -40,11 +40,7 @@ async fn open_period_returns_weeks_and_default_name() {
         .iter()
         .map(|w| w["days"].as_i64().unwrap())
         .collect();
-    assert_eq!(
-        days,
-        vec![6, 7, 7, 7, 4],
-        "Pazartesi başlangıçlı, kısmi haftalar ayrı"
-    );
+    assert_eq!(days, vec![8, 7, 7, 9], "1–8, 9–15, 16–22, 23–31");
 
     let (_, cur) = app.get("/periods/current", &s).await;
     assert_eq!(cur["id"], p["id"]);