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.

backend/app/api/routes/transactions.py

@@ -98,6 +98,19 @@ async def list_transactions(
     return TransactionPage(items=items, total=total, page=page, page_size=page_size)
 
 
+@router.get("/years", response_model=list[int])
+async def list_transaction_years(session: AsyncSession = Depends(get_session)) -> list[int]:
+    """Full range of years to offer in the year picker — from the earliest
+    transaction through the current year, so a gap year with no activity
+    still shows up (as zero) instead of silently disappearing from the UI."""
+    bounds = await session.execute(select(func.min(Transaction.date), func.max(Transaction.date)))
+    min_date, max_date = bounds.one()
+    current_year = date_.today().year
+    if min_date is None:
+        return [current_year]
+    return list(range(min_date.year, max(max_date.year, current_year) + 1))
+
+
 @router.post("", response_model=TransactionRead, status_code=201)
 async def create_transaction(payload: TransactionCreate, session: AsyncSession = Depends(get_session)) -> Transaction:
     await _ensure_category_matches_type(session, payload.category_id, payload.type)