feat: improve recurring cost setup and folder management

docs/TODO.md

@@ -31,6 +31,18 @@ IndexedDB instead of the app.
 
 ---
 
+## Optional monthly income forecast
+
+Add a user-controlled toggle that predicts a month's income from earlier months.
+For each earlier month, divide its recorded income by that month's number of
+workdays; average those per-workday figures, then multiply by the target month's
+workdays. Let the owner adjust the prediction when income changes, for example
+after a raise, without rewriting recorded income or earlier months. Show
+predicted and entered income distinctly, and keep budgeting based on entered
+income unless the owner explicitly chooses otherwise. Before implementation,
+settle whether workdays mean Monday–Friday only or exclude local holidays, and
+whether an adjustment applies to one month or all future predictions.
+
 ## 1. Folder rename blur during an in-flight save
 
 A folder name draft can remain unsaved if the owner changes it while a rename