feat: add Lighthouse IPFS invoice storage integration

[nehayadav827]

Description

Implements decentralized invoice storage through Lighthouse/IPFS.

Changes

• Added src/services/lighthouseService.ts to upload invoice JSON using the Lighthouse REST API.
• Added a Save to IPFS action to the existing Ionic action sheet in Menu.tsx.
• Reads Lighthouse API key from VITE_LIGHTHOUSE_API_KEY.
• Displays the returned CID and Lighthouse gateway URL after successful upload.
• Added VITE_LIGHTHOUSE_API_KEY to .env.example.
• Added Vite environment typing for the new environment variable.

Validation

• npm run build passes successfully.
• git diff --check passes successfully.
• Feature wiring validated through grep-based inspection.

Proof

Attached screenshots include:

1. Scoped diff (git status --short)

2. Successful Vite/build validation using npm run build

3. Lighthouse/IPFS integration wiring proof

Partially addresses #37