[FR] feat(i18n): Add Sinhala (si-LK) language support

### Description

### problem
There is currently no localization for Sinhala (si-LK), which is the native language for over 16 million people in Sri Lanka. Adding Sinhala support would make AppFlowy accessible to a large new demographic of students, professionals, and developers in the region.

### solution
I would like to request adding Sinhala (si-LK) to the supported languages. As a Flutter developer based in Sri Lanka, I am highly familiar with the framework and would love to volunteer to lead the initial translation effort for the core user interface.

If the maintainers can add Sinhala to the project's translation management platform (Crowdin/Weblate), I can begin translating the strings immediately.

### Impact

Adding Sinhala support will significantly lower the barrier to entry for over 20 million native speakers in Sri Lanka. Because AppFlowy excels as an offline-first, privacy-focused productivity tool, it is uniquely positioned to benefit students, local businesses, and professionals in regions where internet connectivity can sometimes be inconsistent.

Furthermore, Sri Lanka has a rapidly growing and highly active open-source tech community. Localizing the application will not only drive user adoption but also encourage more regional developers-especially within the local Flutter community-to discover AppFlowy and contribute back to the codebase.

### Additional Context

Looking forward to contributing to the AppFlowy community!