Fix Excel import and learning startup crashes

[jasonliu1199]

Summary

This PR fixes several stability issues around learning startup and Excel import:

• Ensure the Log directory exists before learning/import flows need it.
• Return early when Excel import fails instead of continuing with a null result.
• Guard Excel parsing against empty rows and empty cells.
• Include the fourth custom-word column when exporting/importing custom content.
• Fix the Japanese word progress lookup by quoting bookName in the SQL query.

Verification

• Ran git diff --check locally.
• Kept this PR limited to code fixes only; no community-fork README, roadmap, or template changes are included.
• The same code changes were included in my fork's maintenance PR and passed the Windows Build workflow: https://github.com/jasonliu1199/ToastFish/actions/runs/27429128508

Related

• Related to win10导入Excel报错 #171: Excel import null reference crash.
• Related to 两个问题：1.邮件“开始”没反应；2.导入单词打不开 #214: import/start failures after downloading v3.0.
• Includes the small SQL fix from 修复：日语单词点记住时闪退 #116.
• Includes the missing Log directory startup protection from 修复因缺失Log文件夹导致随机测试闪退的问题。 #147.