fix: use KB instead of bytes in config file size warning

[LongOddCode]

Fixes #973

Problem

When staticwebapp.config.json exceeds the maximum allowed size (20 KB), the warning message incorrectly displays the unit as "bytes" instead of "KB":

WARNING: staticwebapp.config.json is 22 bytes. The maximum size is 20 bytes.

The values shown (22, 20) are already in kilobytes (the variable is fileSizeInKb and the constant is SWA_RUNTIME_CONFIG_MAX_SIZE_IN_KB), but the label says "bytes".

Fix

Changed the warning message in src/core/utils/user-config.ts to use "KB" instead of "bytes":

WARNING: staticwebapp.config.json is 22 KB. The maximum size is 20 KB.

Testing

Added a test case in src/core/utils/user-config.spec.ts that verifies:

• The warning message contains "KB"
• The warning message does not contain "bytes"

All existing tests continue to pass.