Add support for group income (= negative expenses) (#158)

* Allow negative amount for expenses to be entered

- an expense becomes an income
- this does not affect calculations, i.e. an income can be split just like an expense

* Incomes should not be reimbursements

when entering a negative number
- deselect 'isReimbursement'
- hide reimbursement checkbox

* Change captions when entering a negative number

- "expense" becomes "income"
- "paid" becomes "received"

* Format incomes on expense list

- replace "paid by" with "received by"

* Format incomes on "Stats" tab

- a group's or participants balance might be negative
- in this case "spendings" will be "earnings" (display accordingly)
- always display positive numbers
- for active user: highlight spendings/earnings in red/green

* Fix typo

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Co-authored-by: Sebastien Castiel <sebastien@castiel.me>

src/lib/schemas.ts

@@ -62,7 +62,7 @@ export const expenseFormSchema = z
         ],
         { required_error: 'You must enter an amount.' },
       )
-      .refine((amount) => amount >= 1, 'The amount must be higher than 0.01.')
+      .refine((amount) => amount != 1, 'The amount must not be zero.')
       .refine(
         (amount) => amount <= 10_000_000_00,
         'The amount must be lower than 10,000,000.',