fix expenses list performance + dom nesting

- memoize accordion cards and groupByMonth so toggling one
  expense re-renders O(1) cards instead of the whole list
- cache Intl.NumberFormat and date formatters per key (was
  ~2-3 instantiations per item per render)
- unmount collapsed accordion details via unmountOnExit to cut
  initial mount cost and input delay
- restructure Account row in ExpenseDetail so Chip is not nested
  inside <p> (fixes validateDOMNesting warning)
- avoid mutating expenses state array when sorting

src/Expense/ExpenseDetail.tsx

@@ -33,17 +33,19 @@ export function ExpenseDetail({ item }: { item: ExpenseItem }) {
       <Row
         label="Account"
         value={
-          <Typography variant="body2">
-            {account ? `${account.name}${last4 ? ` (${last4})` : ""}` : "—"}
+          <Box sx={{ display: "flex", alignItems: "center", flexWrap: "wrap", gap: 1 }}>
+            <Typography component="span" variant="body2">
+              {account ? `${account.name}${last4 ? ` (${last4})` : ""}` : "—"}
+            </Typography>
             {account?.type && (
               <Chip
                 size="small"
                 label={account.type.replace(/_/g, " ")}
                 variant="outlined"
-                sx={{ ml: 1.5, fontSize: 11, height: 20 }}
+                sx={{ fontSize: 11, height: 20 }}
               />
             )}
-          </Typography>
+          </Box>
         }
       />
       <Row