Add SDK `submitInvoicesBatch` method

[Levi-Ojukwu]

Description:

The contract supports batch submission of up to 10 invoices in a single transaction. The SDK needs a method that:

1. Validates batch size ≤ 10
2. Authenticates all freelancers in the batch
3. Submits all invoices atomically
4. Returns an array of invoice IDs

Why it matters: Batch submission is essential for high-volume freelancers and platforms.

Acceptance Criteria:

• Create submitInvoicesBatch(client, invoices[]) method
• Validate batch size
• Return array of invoice IDs
• Add tests

Relevant Files: contracts/invoice_liquidity/src/lib.rs:816-912, sdk/src/index.ts

---

[michaelvic123]

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[circleboyslimited]

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