⬆️ chore: Bump TypeScript to 5.9.3 (+ typescript-eslint 8.60.1) (#13584)

Bumps typescript 5.3.3 -> 5.9.3 across all workspaces. typescript-eslint must move 8.24.0 -> 8.60.1 too: 8.24's typescript peer was capped at <5.8.0; 8.60.1 widens it to <6.1.0.

Two errors surfaced by the newer compiler are fixed:
- api/src/rum/proxy.ts: TS 5.9 made `Buffer` generic (`Buffer<ArrayBufferLike>`), which no longer structurally matches `BodyInit`; cast the fetch body (Node's fetch accepts a Buffer at runtime).
- client usePresetIndexOptions.ts: drop a dead `|| {}` on an object spread (always truthy — flagged by the new TS2872 check).

All four package typecheck jobs + the client app typecheck pass under 5.9.3; builds (tsdown + rollup) and the rum proxy tests are unaffected.

packages/api/package.json

@@ -84,7 +84,7 @@
     "rollup-plugin-peer-deps-external": "^2.2.4",
     "sanitize-html": "^2.13.0",
     "ts-node": "^10.9.2",
-    "typescript": "^5.0.4",
+    "typescript": "^5.9.3",
     "xlsx": "https://cdn.sheetjs.com/xlsx-0.20.3/xlsx-0.20.3.tgz",
     "yauzl": "^3.2.1"
   },