feat: add year switcher to Dashboard, Transactions and Budget

Year was previously hardcoded to the current year on all three pages
(useState with no setter) — only the month could be changed via
MonthSelector, making it impossible to look at or compare older years
even though transaction history goes back further than the current
year.

Adds GET /transactions/years (contiguous range from the earliest
transaction through the current year) and a YearSelector component
matching MonthSelector's style, wired into all three pages alongside
a real year useState.

frontend/src/api/transactions.ts

@@ -24,6 +24,13 @@ export function fetchTransactions(filters: TransactionFilters = {}) {
   return api.get<TransactionPage>(`/transactions?${params.toString()}`);
 }
 
+/** Full range of years to offer in the year picker, from the earliest
+ * transaction through the current year (see backend for the "gap year"
+ * rationale). */
+export function fetchTransactionYears() {
+  return api.get<number[]>("/transactions/years");
+}
+
 export function createTransaction(input: TransactionInput) {
   return api.post<Transaction>("/transactions", input);
 }