Add on-device translation to feed posts

[barrydeen]

Summary

• Adds a "Translate" option to the 3-dot overflow menu on every post, using Google ML Kit for on-device translation
• Detects source language automatically, downloads translation models on first use (~30MB per language pair), works offline after that
• Displays translated text inline below the original content with a "Translated from {language}" label
• Menu toggles between "Translate" / "Show Original" / "Show Translation" once translated, shows "Same Language" when source matches device locale
• Translation state cached by event ID via TranslationRepository following the existing Nip05Repository version-counter pattern

Test plan

• Build succeeds with ./gradlew assembleDebug
• Open feed with foreign-language posts, tap 3-dot menu → "Translate"
• Verify progress states show inline (detecting language → downloading model → translating)
• Translated text appears below original with "Translated from {language}" header
• Menu changes to "Show Original" after translation; toggling hides/shows translation
• Same-language posts show disabled "Same Language" menu item
• Translation persists when scrolling away and back
• Translation works on all screens: feed, thread, profile, notifications, search, hashtag feed, bookmark sets