feat(02-01): create useMonthParam hook and MonthNavigator component

- useMonthParam reads/writes month URL search param with YYYY-MM fallback
- navigateMonth handles year rollover via Date constructor
- MonthNavigator renders prev/next arrows with Select dropdown
- Dropdown lists available budget months with locale-aware formatting

src/hooks/useMonthParam.ts

@@ -0,0 +1,26 @@
+import { useSearchParams } from "react-router-dom"
+
+export function useMonthParam() {
+  const [searchParams, setSearchParams] = useSearchParams()
+
+  const monthParam = searchParams.get("month")
+  const now = new Date()
+  const currentMonth = `${now.getFullYear()}-${String(now.getMonth() + 1).padStart(2, "0")}`
+  const month = monthParam || currentMonth
+
+  const setMonth = (newMonth: string) => {
+    setSearchParams((prev) => {
+      prev.set("month", newMonth)
+      return prev
+    })
+  }
+
+  const navigateMonth = (delta: number) => {
+    const [year, mo] = month.split("-").map(Number)
+    const d = new Date(year, mo - 1 + delta, 1)
+    const next = `${d.getFullYear()}-${String(d.getMonth() + 1).padStart(2, "0")}`
+    setMonth(next)
+  }
+
+  return { month, setMonth, navigateMonth }
+}