redesign: YearSelector as a compact dropdown next to MonthSelector

The pill-row year switcher from the previous commit added a whole
extra row of buttons above the month pills, which read as cluttered.
Replaced it with a single dropdown button (current year + chevron)
placed to the right of MonthSelector on all three pages, matching how
a lot of finance-app date pickers separate month (frequent) from year
(occasional) navigation.

frontend/src/pages/BudgetPage.tsx

@@ -54,9 +54,11 @@ export function BudgetPage() {
     <div className="space-y-5">
       <AlertBanner />
 
-      <div className="space-y-2">
+      <div className="flex items-center gap-3">
+        <div className="min-w-0 flex-1">
+          <MonthSelector month={month} onChange={setMonth} />
+        </div>
         <YearSelector years={years ?? [now.getFullYear()]} year={year} onChange={setYear} />
-        <MonthSelector month={month} onChange={setMonth} />
       </div>
 
       <Card>

frontend/src/pages/DashboardPage.tsx

@@ -35,9 +35,11 @@ export function DashboardPage() {
     <div className="space-y-5">
       <AlertBanner excludeKeys={["risky_allocation_exceeded"]} />
 
-      <div className="space-y-2">
+      <div className="flex items-center gap-3">
+        <div className="min-w-0 flex-1">
+          <MonthSelector month={month} onChange={setMonth} />
+        </div>
         <YearSelector years={years ?? [now.getFullYear()]} year={year} onChange={setYear} />
-        <MonthSelector month={month} onChange={setMonth} />
       </div>
 
       {isError && (

frontend/src/pages/TransactionsPage.tsx

@@ -63,7 +63,16 @@ export function TransactionsPage() {
   return (
     <div className="space-y-5">
       <div className="flex flex-col gap-3 sm:flex-row sm:items-center sm:justify-between">
-        <div className="space-y-2">
+        <div className="flex min-w-0 flex-1 items-center gap-3">
+          <div className="min-w-0 flex-1">
+            <MonthSelector
+              month={month}
+              onChange={(value) => {
+                setMonth(value);
+                setPage(1);
+              }}
+            />
+          </div>
           <YearSelector
             years={years ?? [now.getFullYear()]}
             year={year}
@@ -72,13 +81,6 @@ export function TransactionsPage() {
               setPage(1);
             }}
           />
-          <MonthSelector
-            month={month}
-            onChange={(value) => {
-              setMonth(value);
-              setPage(1);
-            }}
-          />
         </div>
         <Button onClick={openCreateModal} className="w-full sm:w-auto">
           <Plus size={16} />