fix(tests): fix EBUSY crash on Windows, correct Unicode receipt assertions, graceful printer skip

[khaira777]

Summary

Three test-suite fixes found while running npm test on Windows for the first time.

Fixes

1. tests/backup-restore.test.ts — EBUSY crash on Windows

• createTestDb() was returning a new open DatabaseSync handle after already closing the DB.
• This leaked an open SQLite connection that Windows locked, causing fs.rmSync() to throw EBUSY: resource busy or locked on cleanup.
• Fix: Changed return type to void and added currentDb.close() before cleanup().
• macOS/Linux were unaffected (OS releases file handles more loosely), which is why this wasn't caught before.

2. tests/printer.test.ts — Wrong currency in assertions

• All formatReceipt() calls in tests were missing useUnicode: true.
• Without it, formatReceipt maps ₹ → Rs (ASCII fallback for old thermal printers), so assertions like text.includes('₹925.00') were always false.
• This caused 12 out of 58 tests to fail on all platforms.
• Fix: Added useUnicode: true to all formatReceipt() calls in the test suite.

3. tests/printer.test.ts — Hard failure when no printer driver installed

• The "host has at least one printer installed" assertion was a hard failure on any machine without thermal printer drivers (dev machines, CI).
• Fix: Changed to a graceful informational skip when no printers are detected.

Testing

• All 58 printer tests now pass ✅
• All 4 backup/restore tests now pass ✅
• DB audit passes ✅
• Tested on Windows 11