Adds per-user wallet configuration

Allows users to configure their own wallet ID, enabling
the system to track expenses and receivables on a per-user basis.

Introduces new database table, models, API endpoints, and UI elements
to manage user-specific wallet settings.

models.py

@@ -119,3 +119,16 @@ class UserCastleSettings(CastleSettings):
     """User-specific settings (stored with user_id)"""
 
     id: str
+
+
+class UserWalletSettings(BaseModel):
+    """Per-user wallet settings"""
+
+    user_wallet_id: Optional[str] = None  # The wallet ID for this specific user
+    updated_at: datetime = Field(default_factory=lambda: datetime.now())
+
+
+class StoredUserWalletSettings(UserWalletSettings):
+    """Stored user wallet settings with user ID"""
+
+    id: str  # user_id