feat(panel): harcanabilir ve grup harcama grafikleri

- Günlük / Haftalık / Aylık sekmeli iki çizgi grafik (Recharts, ayrı parça olarak yüklenir)
- Kümülatif harcanabilir vs gerçekleşen; 4 grup (+ isteğe bağlı harcanabilir çizgisi)
- Dönem istatistiklerine grup kırılımı (spent_by_group)
- Grafik altında metin özeti, koyu tema uyumlu renkler

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

backend/tests/stats.rs

@@ -123,7 +123,9 @@ async fn period_stats_and_total_savings() {
     let s = app.register("Test", None).await;
     let (nov, dec) = two_periods(&app, &s).await;
     let needs = category(&app, &s, "needs").await;
+    let culture = category(&app, &s, "culture").await;
     spend(&app, &s, &dec, "2026-12-05", 40_000, &needs).await;
+    spend(&app, &s, &dec, "2026-12-06", 2_500, &culture).await;
 
     let (st, b) = app.get("/stats/periods", &s).await;
     assert_eq!(st, StatusCode::OK, "{b}");
@@ -135,6 +137,10 @@ async fn period_stats_and_total_savings() {
     assert_eq!(list[0]["status"], "closed");
     assert_eq!(list[0]["spendable"], 1_000_000);
     assert_eq!(list[0]["spent"], 300_000);
+    assert_eq!(
+        list[0]["spent_by_group"],
+        json!({ "needs": 300_000, "wants": 0, "culture": 0, "unexpected": 0 })
+    );
     assert_eq!(list[0]["to_savings"], 500_000);
     assert_eq!(list[0]["last_date"], "2026-11-30");
     assert_eq!(list[1]["id"], dec.as_str());
@@ -142,7 +148,9 @@ async fn period_stats_and_total_savings() {
         list[1]["spendable"], 2_100_000,
         "20.000 − 1.000 hedef + 2.000 artı"
     );
-    assert_eq!(list[1]["spent"], 40_000);
+    assert_eq!(list[1]["spent"], 42_500);
+    assert_eq!(list[1]["spent_by_group"]["needs"], 40_000);
+    assert_eq!(list[1]["spent_by_group"]["culture"], 2_500);
     assert_eq!(list[1]["to_savings"], Value::Null);
 
     // limit: en yeni N dönem