Fix all 20 pre-existing type errors; typecheck now green

- ui/chart.tsx: recharts 3 stopped exposing tooltip/legend content props
  on its public types; declare the ChartPayloadItem shape we consume
- base-ui Select onValueChange passes string|null: guard null in
  BillingCycleSelector, PasteImportModal, TransactionModal
- type the untyped api.get calls (cycles, categories)
- push-notifications: back the VAPID key with an explicit ArrayBuffer
  so it satisfies BufferSource under TS 5.9

Unblocks gating CI on pnpm typecheck.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/src/components/PasteImportModal.tsx

@@ -61,7 +61,7 @@ export default function PasteImportModal({ onClose, onImported }: Props) {
             <div className="grid grid-cols-2 gap-4">
               <div className="space-y-2">
                 <Label>Bank</Label>
-                <Select value={bank} onValueChange={setBank}>
+                <Select value={bank} onValueChange={(v) => v && setBank(v)}>
                   <SelectTrigger className="w-full">
                     <SelectValue />
                   </SelectTrigger>
@@ -74,7 +74,7 @@ export default function PasteImportModal({ onClose, onImported }: Props) {
               </div>
               <div className="space-y-2">
                 <Label>Source</Label>
-                <Select value={source} onValueChange={setSource}>
+                <Select value={source} onValueChange={(v) => v && setSource(v)}>
                   <SelectTrigger className="w-full">
                     <SelectValue />
                   </SelectTrigger>