Add Bengali language support

[SifatAlMukit]

Summary

This pull request adds Bengali (bn) language support to Atomic - Periodic Table.

Changes

• Added Bengali element data in app/src/main/assets/elements_bn.json
• Added Bengali Android UI strings in app/src/main/res/values-bn/strings.xml
• Added Bengali to the language selector in SettingsActivity.kt

Validation

• The element JSON is valid UTF-8 JSON and contains all 118 elements
• Element and field ordering matches the English source
• element_phase remains unchanged as Solid, Liquid, or Gas
• Android string placeholders and escape sequences were preserved
• The Bengali XML resource contains all current translatable keys

Notes

The translation uses standard Bengali scientific terminology commonly understood in Bangladesh and West Bengal.

[JLindemann42]

Hello!

Thanks for the great work. Will look into your translations and merge it with the app soon

Thanks again,
Jonatan Lindemann