fix(frontend): add global Window typings for Google Drive Picker integration

[Ludy87]

Description of Changes

What was changed

• Added a new TypeScript declaration file: frontend/src/core/types/googleDrivePicker.d.ts.
• Extended the global Window interface to include:
  • window.gapi: typeof gapi
  • window.google: typeof google

Why the change was made

• Fixes TypeScript compilation errors (TS2339) where window.gapi and window.google were used by googleDrivePickerService.ts, but were not declared on Window & typeof globalThis.
• Ensures the Google Drive Picker integration can access the expected global objects with proper typings.

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Checklist

General

• I have read the Contribution Guidelines
• I have read the Stirling-PDF Developer Guide (if applicable)
• I have read the How to add new languages to Stirling-PDF (if applicable)
• I have performed a self-review of my own code
• My changes generate no new warnings

Documentation

• I have updated relevant docs on Stirling-PDF's doc repo (if functionality has heavily changed)
• I have read the section Add New Translation Tags (for new translation tags only)

Translations (if applicable)

• I ran scripts/counter_translation.py

UI Changes (if applicable)

• Screenshots or videos demonstrating the UI changes are attached (e.g., as comments or direct attachments in the PR)

Testing (if applicable)

• I have tested my changes locally. Refer to the Testing Guide for more details.

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