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/lib/push-notifications.ts

@@ -1,10 +1,11 @@
 import api from './api';
 
-function urlBase64ToUint8Array(base64String: string): Uint8Array {
+function urlBase64ToUint8Array(base64String: string): Uint8Array<ArrayBuffer> {
   const padding = '='.repeat((4 - (base64String.length % 4)) % 4);
   const base64 = (base64String + padding).replace(/-/g, '+').replace(/_/g, '/');
   const rawData = window.atob(base64);
-  const outputArray = new Uint8Array(rawData.length);
+  // Explicit ArrayBuffer backing so the result satisfies BufferSource.
+  const outputArray = new Uint8Array(new ArrayBuffer(rawData.length));
   for (let i = 0; i < rawData.length; ++i) {
     outputArray[i] = rawData.charCodeAt(i);
   }