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/models/models.py

@@ -140,6 +140,7 @@ class TransactionBase(SQLModel):
     bank: Bank = Bank.BAC
     notes: Optional[str] = None
     category_id: Optional[int] = Field(default=None, foreign_key="category.id")
+    deferred_to_next_cycle: bool = Field(default=False)
 
 
 class Transaction(TransactionBase, table=True):
@@ -168,6 +169,7 @@ class TransactionUpdate(SQLModel):
     source: Optional[TransactionSource] = None
     notes: Optional[str] = None
     category_id: Optional[int] = None
+    deferred_to_next_cycle: Optional[bool] = None
 
 
 # --- Exchange Rate ---