fix(security): fix npm audit (#7824)

# Motivation

Npm detected a security vulnerability:

```
@sveltejs/kit  <=2.57.0
Severity: high
@sveltejs/kit: Unvalidated redirect in handle hook causes Denial-of-Service - GHSA-3f6h-2hrp-w5wx
@sveltejs/adapter-node has a BODY_SIZE_LIMIT bypass - GHSA-2crg-3p73-43xp
fix available via `npm audit fix`
```

# Changes

Run npm audit fix + fix TS new detection.

# Tests

Tests should pass.

# Todos

- [x] Accessibility (a11y) – any impact?
- [x] Changelog – is it needed?

Author: 0xFFrancesco

frontend/package-lock.json

@@ -7,3 +7,14 @@ declare namespace App {
   // interface Error {}
   // interface Platform {}
 }
+
+/** WICG File System Access — TypeScript's bundled `lib.dom` does not declare `showSaveFilePicker` on `Window`. */
+interface Window {
+  showSaveFilePicker?: (options: {
+    suggestedName?: string;
+    types?: Array<{
+      description: string;
+      accept: Record<string, string[]>;
+    }>;
+  }) => Promise<FileSystemFileHandle>;
+}

frontend/src/app.d.ts

@@ -23,7 +23,7 @@ const saveFileWithPicker = async ({
   description: string;
 }) => {
   try {
-    const handle = await window.showSaveFilePicker({
+    const handle = await window.showSaveFilePicker!({
       suggestedName: `${fileName}.csv`,
       types: [
         {