Budget: show planned surplus/deficit (expected income − budgeted spending)

[sairus2k]

Describe the feature

The budget page never compares expected income with budgeted spending, so a month planned at a deficit looks exactly like a balanced one. Every workaround (budgeting below real spending, planning income in categories, negative amounts) breaks the donut or flags categories as over budget for the whole month.

Proposal: add a Planned surplus / deficit line (expected income − budgeted spending) to the budgeted summary, and keep that summary visible on fully allocated budgets (available_to_allocate >= 0 instead of > 0 in budgets/show.html.erb).

Why is this feature important?

A deficit month should be a deliberate decision ("we cover it from savings"), not something the app hides once allocation is done.

Additional context, screenshots, and relevant links

• Bug: Income shouldn't appear in category budgets #1335: why planning income in budget categories doesn't work as a workaround (they are treated as spending envelopes). Its proposal to plan expected income by source would be a natural place for this line.
• Budget shows two different "what's left" figures with no stated relationship #3218: a similar gap between plan and cash on hand. This request is about plan vs expected income, so it isn't a duplicate and needs no model change.