feat(ui): motion that answers the finger, and honours reduced-motion; release v6.2.0

The mobile menu becomes a surface you drag rather than one that appears and
vanishes: it tracks the finger, carries its velocity when flicked, can be caught
mid-flight, and resists past the edge instead of stopping dead. Buttons react on
press rather than on release, and the browser's tap delay is gone — nothing here
zooms, so the wait bought nothing.

prefers-reduced-motion, prefers-reduced-transparency and prefers-contrast are
now respected instead of ignored: looping decoration stops, transitions fade
rather than slide, the chat jumps to the newest message, frosted panels turn
solid and edges get drawn. What stops is the movement, never the information —
recording indicators and other status stay.

Also bumps the desktop download buttons to 0.5.0. The version lives in one
constant per source file and tests/desktop-download-links.test.mjs fetches every
generated URL, so the links are proven to serve a real asset rather than merely
looking well-formed.

Claude-Session: https://claude.ai/code/session_01ARFZ9G6P1e1B4w75kFaeNw

doc/CRYPTOGRAPHY.md

@@ -7,7 +7,7 @@ this document describes.
 
 | | |
 | --- | --- |
-| Release | v6.1.1 |
+| Release | v6.2.0 |
 | Protocol version | 4.1 |
 | Ratchet wire version | 1 |
 
@@ -308,5 +308,5 @@ and receiving their group traffic.
 
 ## Scope
 
-This describes the browser implementation as it stands in v6.1.1. It is not a
+This describes the browser implementation as it stands in v6.2.0. It is not a
 substitute for independent cryptographic review.