Add deferred transactions, revamp budget projections and UI

Adds deferred_to_next_cycle flag to transactions for billing cycle
bleed-over handling. Overhauls budget projection engine and refreshes
Budget page with improved monthly detail and transaction columns.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/api/v1/endpoints/budget.py

@@ -194,6 +194,11 @@ class ActualsBySource(BaseModel):
     count: int
 
 
+class CCCategorySpending(BaseModel):
+    category_name: str
+    amount: float
+
+
 class MonthlyDetailResponse(BaseModel):
     year: int
     month: int
@@ -207,6 +212,7 @@ class MonthlyDetailResponse(BaseModel):
     uncovered_actual: float
     gran_total_egresos: float
     net_balance: float
+    cc_by_category: list[CCCategorySpending]
 
 
 @router.get("/month/{year}/{month}", response_model=MonthlyDetailResponse)
@@ -230,6 +236,7 @@ def get_monthly_detail(
         uncovered_actual=data["uncovered_actual"],
         gran_total_egresos=data["gran_total_egresos"],
         net_balance=data["net_balance"],
+        cc_by_category=[CCCategorySpending(**c) for c in data["cc_by_category"]],
     )