docs(translation): document on-device message translation

Add the feature to README (Messaging) and LOTUS_FEATURES (new
On-Device Message Translation section under Messaging Enhancements),
matching the existing style.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -36,6 +36,7 @@ The Lotus Chat logo (`public/res/Lotus.png`) is a derivative work based on the o
 - Control voice message playback speed: 0.75× / 1× / 1.5× / 2×
 - Search messages with a date range filter
 - Optional persistent search index for encrypted rooms (off by default — stores decrypted text on your device; clearable, wiped on logout)
+- On-device message translation — foreign-language messages show a "Translate" action, then an inline "Translated from <language> · Show original" toggle. Runs entirely on your device via the browser's built-in translation models, so message text never leaves your device or touches a cloud service (no Google/DeepL/Microsoft) — preserving end-to-end encryption. Pick your target language and optionally auto-translate incoming messages at Settings → General → Messages. Chromium desktop (Chrome/Edge 138+) and the Lotus desktop app only; hidden where unavailable (Firefox, Safari, mobile)
 - Write math with LaTeX: `$inline$` and `$$block$$` render via KaTeX (spec `data-mx-maths` supported)
 - Room topics support rich formatting (bold, links, italics)
 - Deleted messages show a placeholder instead of disappearing