docs: update the swipe limits and note what pauses or replaces a swipe

Merge-time fix for #674: the mobile-handlers.js @fileoverview still
gave the old 300 ms and 100 px limits (now 500 ms and 50 px), and the
Mobile Guide now says a visible text selection pauses the swipe and a
terminal wider than the screen pans instead of switching.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/web/public/mobile-handlers.js

@@ -9,7 +9,7 @@
  *   toolbar/accessory bar repositioning, terminal resize on keyboard open/close,
  *   and input scroll-into-view. Uses 100px threshold for iOS address bar drift.
  * - SwipeHandler — Horizontal swipe detection on the terminal area for session switching.
- *   80px minimum distance, 300ms maximum time, 100px max vertical drift.
+ *   80px minimum distance, 500ms maximum time, 50px max vertical drift.
  *
  * All three have init()/cleanup() lifecycle methods. They are re-initialized after SSE
  * reconnect (in handleInit) to prevent stale closures.