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

---------

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

src/app/groups/[groupId]/stats/totals-your-share.tsx

@@ -1,7 +1,7 @@
 'use client'
 import { getGroup, getGroupExpenses } from '@/lib/api'
 import { getTotalActiveUserShare } from '@/lib/totals'
-import { formatCurrency } from '@/lib/utils'
+import { cn, formatCurrency } from '@/lib/utils'
 import { useEffect, useState } from 'react'
 
 type Props = {
@@ -26,8 +26,13 @@ export function TotalsYourShare({ group, expenses }: Props) {
   return (
     <div>
       <div className="text-muted-foreground">Your total share</div>
-      <div className="text-lg">
-        {formatCurrency(currency, totalActiveUserShare)}
+      <div
+        className={cn(
+          'text-lg',
+          totalActiveUserShare < 0 ? 'text-green-600' : 'text-red-600',
+        )}
+      >
+        {formatCurrency(currency, Math.abs(totalActiveUserShare))}
       </div>
     </div>
   )