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>

LOTUS_FEATURES.md

@@ -1,7 +1,7 @@
 # Lotus Chat — Feature Reference
 
 Everything added to Lotus Chat beyond upstream Cinny v4.12.1.
-Last updated: June 2026.
+Last updated: July 2026.
 
 ---
 
@@ -698,6 +698,41 @@ Context menu → **Forward** allows forwarding a message to any room the user is
 
 Context menu → **Copy Text** copies a message's plain-text body to the clipboard (reply fallback stripped via `trimReplyFromBody`), complementing the existing **Copy Link** (permalink) action. It renders only when the event has a usable text body, so media without a caption doesn't show an empty action.
 
+### On-Device Message Translation
+
+Translate chat messages written in other languages into a language you choose,
+inline in the timeline — running **entirely on your device** so message text
+never leaves it.
+
+- **Per-message translate** — a foreign-language message shows a **Translate**
+  action in its message menu; once translated, the message displays an inline
+  **"Translated from <language> · Show original"** toggle that swaps between
+  the translation and the original text.
+- **Fully on-device / E2EE-preserving** — translation and language detection run
+  through the browser's built-in **Translator** and **Language Detector** APIs
+  (the Chromium on-device AI translation models). Message text is **never** sent
+  to any cloud translation service — no Google / DeepL / Microsoft, not even a
+  self-hosted server — and there is **no network fallback**, so end-to-end
+  encryption is preserved. That privacy guarantee is the whole point of the
+  feature.
+- **Automatic detection** — the language of each message is detected
+  automatically; messages already in your target language are skipped (no
+  Translate action is shown).
+- **Settings (Settings → General → Messages):**
+  - **Translate Messages Into** — your target language (default **English**;
+    ~26 common languages).
+  - **Auto-translate Incoming Messages** (default **off**) — automatically
+    translates foreign-language messages whose on-device language model is
+    already downloaded.
+- **One-time model download** — the first time you translate from a given
+  language, a small on-device model (a few MB) downloads once. Because the
+  browser requires a user gesture for that first download, the initial
+  translation needs a click.
+- **Availability** — Chromium desktop browsers (**Chrome / Edge 138+**) and the
+  **Lotus desktop app** (WebView2 / Chromium). Not available in Firefox, Safari,
+  or mobile browsers; where the APIs are unavailable the feature hides itself and
+  the settings tile shows a note.
+
 ### Draft Persistence
 
 - Composer drafts are stored in `localStorage` keyed by `roomId`