feat: show per-group report stats in collapsed accordion header

Move the Sum/Count/Avg/Min/Max/First/Last metric row from inside the
expanded accordion details into the accordion summary as a second row,
so group stats are visible while the group is collapsed. Second row
renders compact label-value pairs instead of pills; summary content is
now a two-row column.

src/common/components/TransactionList.tsx

@@ -165,12 +165,14 @@ export function TransactionList({ items, fields, granularity = "monthly", showMe
                   px: 2,
                   py: 1,
                   "& .MuiAccordionSummary-content": {
-                    alignItems: "center",
+                    flexDirection: "column",
-                    gap: 1.5,
+                    alignItems: "stretch",
+                    gap: 0.75,
                     minWidth: 0,
                   },
                 }}
               >
+                <Box sx={{ display: "flex", alignItems: "center", gap: 1.5, minWidth: 0 }}>
                   <Typography variant="h6" fontWeight={700} sx={{ letterSpacing: "-0.01em" }}>
                     {group.label}
                   </Typography>
@@ -187,9 +189,12 @@ export function TransactionList({ items, fields, granularity = "monthly", showMe
                   <Typography variant="body2" fontWeight={700} color="success.main">
                     {formatCurrency(group.income, group.currency)}
                   </Typography>
+                </Box>
+                <Box sx={{ display: "flex", alignItems: "stretch", gap: 1.5, minWidth: 0 }}>
+                  {showMetrics && <GroupMetrics items={group.items} currency={group.currency} />}
+                </Box>
               </AccordionSummary>
               <AccordionDetails sx={{ p: 1.5, pt: 1 }}>
-                {showMetrics && <GroupMetrics items={group.items} currency={group.currency} />}
                 <Box sx={{ display: "flex", flexDirection: "column", gap: 1 }}>
                   {groupByDate(group.items).map((dateGroup) => (
                     <Box