AxeOS UI: add i18n with DE/ES translations and language selector

[marsmensch]

Summary

• Add a lightweight i18n service + translate pipe with JSON locale bundles (EN/DE/ES) and English fallback.
• Replace AxeOS UI strings with stable i18n keys across Dashboard, Settings, Network, Pool, Update, Logs, and Swarm.
• Add Settings → UI language selector (persisted; applied on Save).
• Localize share rejection labels and tooltip explanations.
• Add contributor docs and an i18n:check script for locale validation.

Testing

• npm run i18n:check
• npm test (Node 22 LTS)

Notes

• Language changes apply only after clicking Save (consistent with existing Settings behavior).
• Scope limited to AxeOS web UI; no mining/firmware logic changes.

[0xf0xx0]

code ack, testing

[0xf0xx0]

tested and works for me, good change :3