feat(cli): add partial payment support and preview to iln pay command…

[BernardOnuh]

What this PR does

Implements the iln pay command with partial payment mode and a payment preview before signing — resolving the two missing features called out in #232.

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Usage

# Full payment (defaults to remaining balance)
iln pay --id INV-100

# Partial payment
iln pay --id INV-100 --amount 50

# Skip confirmation
iln pay --id INV-100 --amount 50 --yes

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User-facing output

Preview (shown before signing):
Paying 50 USDC of 100 USDC remaining on Invoice #INV-100 (Payer earns LP: 0.25 USDC)

Partial progress:
Paid 50 USDC. 50 USDC remaining.

Settlement receipt (full payment):
── Settlement Receipt ──────────────────────────────

Invoice: INV-100

Paid: 100 USDC

Remaining: 0 USDC

LP earned: 0.5 USDC

Tx hash: TX_ABC123

Status: FULLY PAID ✓

────────────────────────────────────────────────────

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Files changed

File	What
cli/src/commands/pay-types.ts	PayOptions, InvoicePayState, PayResult types
cli/src/commands/pay.ts	Command factory + preview/receipt printers
cli/tests/e2e/pay.test.ts	14 tests across all flows
cli/src/index.ts	Wired makePayCommand into the CLI

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Test coverage

Scenario	Covered
Full payment calls executor with remaining balance	✅
Partial payment calls executor with --amount value	✅
Amount exceeding remaining balance rejected with error	✅
Preview printed before signing	✅
LP earnings note shown / hidden based on invoice	✅
Confirmation prompt shown without --yes	✅
Prompt skipped with --yes	✅
Payment cancelled on user decline	✅
Settlement receipt on full payment	✅
Partial progress line on partial payment	✅

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Closes #232

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