feat: add Welsh translations

[danryland]

Summary

• add Welsh (cy) to the supported language list as Cymraeg
• register the Welsh locale bundle
• translate unsubscribe, re-subscribe, preference management, snoozing, and email footer copy using informal Welsh
• encode the Welsh flag with explicit Unicode escapes so it renders correctly without triggering GitHub hidden-Unicode warnings

Validation

• yarn workspace @plunk/shared build
• Prettier check for all three changed files
• locale key parity with English: 44 of 44 keys
• interpolation placeholder parity with English
• verified the escaped flag evaluates to the complete Wales emoji sequence
• git diff --check

[driaug]

Is there a specific technical reason you encoded the Welsh flag?

If it is just to get rid of the hidden character warning from GitHub then there is no need, you may just use the emoji directly.

[danryland]

@driaug Yes, it was to get rid of the Github warning. Reverted to emoji flag now. Thanks!