style(stats): add hover state and hint text to chart legend labels (#461)

Legend labels now show a subtle background on hover and a
"Click a label to show or hide it" hint below each chart.

src/components/stats/cumulative-chart.tsx

···
             }}
             formatter={(value, entry) => (
               <span
-                className="cursor-pointer select-none text-sm"
+                className="cursor-pointer select-none rounded px-1 py-0.5 text-sm transition-colors hover:bg-muted"
                 style={{
                   color: hiddenGroups.has(String(entry.dataKey))
                     ? 'var(--muted-foreground)'
···
           })}
         </AreaChart>
       </ResponsiveContainer>
+      <p className="mt-1 text-center text-xs text-muted-foreground">
+        Click a label to show or hide it
+      </p>
     </div>
   );
 }

src/components/stats/daily-chart.tsx

···
             }}
             formatter={(value, entry) => (
               <span
-                className="cursor-pointer select-none text-sm"
+                className="cursor-pointer select-none rounded px-1 py-0.5 text-sm transition-colors hover:bg-muted"
                 style={{
                   color: hiddenGroups.has(String(entry.dataKey))
                     ? 'var(--muted-foreground)'
···
           })}
         </BarChart>
       </ResponsiveContainer>
+      <p className="mt-1 text-center text-xs text-muted-foreground">
+        Click a label to show or hide it
+      </p>
     </div>
   );
 }