docs: reconcile tab-title + collapsible-message threshold claims with code

Two bug-hunt findings that were doc inaccuracies, not code bugs:
- Tab title "(N)" is the mention/highlight count (+ "·" for other unread),
  mirroring the favicon — intentional. LOTUS_FEATURES said "N unread messages";
  corrected to describe the actual highlight-count behavior.
- Collapsible long messages use a fixed COLLAPSE_MAX_HEIGHT (320px ≈ 20 lines);
  the doc claimed a Settings → Appearance control that never existed. Corrected
  to describe the fixed threshold rather than build a marginal per-user setting.

Verified against ClientNonUIFeatures.tsx and MsgTypeRenderers.tsx.

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

LOTUS_FEATURES.md

@@ -1019,10 +1019,12 @@ Fixed by replacing the single read with a `readStatus()` function called inside
 
 The browser tab title updates to reflect unread state:
 
-- `(N) Lotus Chat` — N unread messages
-- `· Lotus Chat` — unread activity without a specific count
+- `(N) Lotus Chat` — N mentions / keyword highlights (the count is highlights, not total unread)
+- `· Lotus Chat` — unread messages without a mention (activity, no specific count)
 - `Lotus Chat` — no unread items
 
+The favicon mirrors this (highlight badge / unread dot / default).
+
 ### Extended Profile Fields
 
 Supports MSC4133 custom profile fields via `PUT /_matrix/client/unstable/uk.tcpip.msc4133/{userId}/{field}`:
@@ -1094,10 +1096,9 @@ OS-level notifications are unchanged and still fire when the window is not focus
 
 ### Collapsible Long Messages
 
-Messages exceeding a configurable line threshold are truncated with a "Show more" toggle.
+Messages exceeding a fixed height threshold are truncated with a "Show more" toggle.
 
-- Default threshold: 20 lines
-- Threshold is configurable in **Settings → Appearance**
+- Threshold: a fixed `COLLAPSE_MAX_HEIGHT` of 320px (≈ 20 lines) — not currently user-configurable
 - Uses CSS `max-height` + `overflow: hidden` with a smooth transition
 - Transition is disabled when `prefers-reduced-motion: reduce` is active