Fix chart legend order and tooltip stacking on wage graphs

Legend was sorted to mirror bar stack order, making it hard to tell
segments apart at a glance; reverse it. Tooltip also rendered behind
the legend div when they overlapped — raise its z-index above it.

frontend/src/pages/Rolling12Months.tsx

@@ -151,7 +151,7 @@ export default function Rolling12Months() {
                 <XAxis dataKey="label" tick={{ fontSize: 10 }} />
                 <YAxis tickFormatter={v => `£${Math.round(v / 1000)}k`} tick={{ fontSize: 11 }} width={55} />
                 <Tooltip formatter={(v: unknown) => fmtMoney(Number(v))} />
-                <Legend />
+                <Legend itemSorter={item => -deptCols.findIndex(dep => dep.name === item.dataKey)} />
                 {deptCols.map(dep => (
                   <Bar key={dep.id} dataKey={dep.name} stackId="a" fill={dep.color} />
                 ))}