feat(04-02): add locale prop and custom currency tooltips to chart components

- Add locale?: string prop to ExpenseBreakdown and AvailableBalance
- Replace bare <Tooltip /> in ExpenseBreakdown with custom content renderer
- Add Tooltip import and custom content renderer to AvailableBalance
- Pass locale to formatCurrency in AvailableBalance center text
- Tooltip styled with shadcn design tokens (bg-background, border-border/50, shadow-xl)

frontend/src/components/AvailableBalance.tsx

@@ -1,6 +1,6 @@
 import { useTranslation } from 'react-i18next'
 import { Card, CardContent, CardHeader, CardTitle } from '@/components/ui/card'
-import { PieChart, Pie, Cell, ResponsiveContainer } from 'recharts'
+import { PieChart, Pie, Cell, ResponsiveContainer, Tooltip } from 'recharts'
 import type { BudgetDetail } from '@/lib/api'
 import { formatCurrency } from '@/lib/format'
 import { palette, headerGradient, type CategoryType } from '@/lib/palette'
@@ -8,9 +8,10 @@ import { cn } from '@/lib/utils'
 
 interface Props {
   budget: BudgetDetail
+  locale?: string
 }
 
-export function AvailableBalance({ budget }: Props) {
+export function AvailableBalance({ budget, locale = 'en' }: Props) {
   const { t } = useTranslation()
   const { totals } = budget
 
@@ -48,11 +49,25 @@ export function AvailableBalance({ budget }: Props) {
                   <Cell key={index} fill={palette[entry.categoryType]?.base ?? palette.carryover.base} />
                 ))}
               </Pie>
+              <Tooltip
+                content={({ active, payload }) => {
+                  if (!active || !payload?.length) return null
+                  const item = payload[0]
+                  return (
+                    <div className="rounded-lg border border-border/50 bg-background px-2.5 py-1.5 text-xs shadow-xl">
+                      <p className="font-medium text-foreground">{item.name}</p>
+                      <p className="font-mono tabular-nums text-muted-foreground">
+                        {formatCurrency(Number(item.value), budget.currency, locale)}
+                      </p>
+                    </div>
+                  )
+                }}
+              />
             </PieChart>
           </ResponsiveContainer>
           <div className="absolute inset-0 flex flex-col items-center justify-center">
             <span className={cn('text-3xl font-bold tabular-nums', available >= 0 ? 'text-success' : 'text-destructive')}>
-              {formatCurrency(available, budget.currency)}
+              {formatCurrency(available, budget.currency, locale)}
             </span>
             <span className="text-xs text-muted-foreground">{t('dashboard.available', 'Available')}</span>
           </div>