Fix Gemini API 404 Error by Migrating to Latest @google/genai SDK

[AsadAhmedSaiyed]

Fixes #146

• Description:
  This PR fixes the issue where receipt processing using the Gemini API failed with a 404 error when using the gemini-1.5-flash-latest model.

• Problem:
  The previous implementation used the old SDK @google/generative-ai, which defaults to the v1beta API. This caused errors like "gemini-1.5-latest model not found".

• Solution:

1. Migrated the project to the latest @google/genai SDK which uses the correct /v1 endpoint.
2. Updated imports and client initialization according to the Gemini API Quickstart Docs
3. Updated all generateContent() calls to use the new SDK.
4. Ensured receipt uploads now successfully generate AI-extracted data.

• Testing:

1. npm test gives positive output.
2. Receipt processing now works without errors.

• Impact:

1. Fixes the 404 error and ensures full compatibility with Gemini 1.5 Flash.
2. Receipt processing and transaction creation are now reliable.

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[archa8]

Issue is fixed. LGTM!