feat(player): add avatar-centered drag steering

docs/UI_STYLE.md

@@ -64,14 +64,16 @@ are not MVP requirements.
 - Visible keyboard-focus outlines are not required for the MVP.
 - On computers, movement works from the keyboard and from an on-screen
   directional control cluster fixed in the lower-right corner.
-- On touch devices, movement uses an always-visible circular joystick at least
-  160 by 160 CSS pixels. Its drag stays within the control, returns to center
-  on release, cancellation, pause, disconnect, and browser blur, and sends
-  only cardinal movement intentions.
-- The joystick has a dead zone and directional hysteresis. Its locally
-  scheduled repeat interval is bounded and never faster than the server's
-  movement cooldown. Touch swipes remain supported. Phones do not show a
-  directional-button cluster.
+- On touch devices, the centered player avatar is the movement handle. Its
+  transparent touch target is at least 96 by 96 CSS pixels and its visible
+  character remains unobscured. During a drag, it deflects no more than 28 CSS
+  pixels from center and returns exactly to center on release, cancellation,
+  pause, disconnect, lost pointer capture, hidden document, and browser blur.
+- The avatar drag has a 12 CSS-pixel dead zone and directional hysteresis. Its
+  bounded repeat cadence changes monotonically from 360 ms at a small
+  intentional displacement to 170 ms at the target edge; it never exceeds the
+  server's 150 ms movement cooldown. Touch swipes remain supported. Phones do
+  not show a directional-button cluster.
 - The always-visible key-drop control is labelled `OK`, includes a key icon,
   and is positioned closer to the upper edge of the screen than the
   directional controls. Dropping does not require confirmation.
@@ -160,7 +162,7 @@ are not MVP requirements.
 ## 12. Pre-reader onboarding and usability
 
 - On a player's first active projection, show an eight-step, wordless-first
-  walkthrough for joystick drag and release, walls, key, vision, compass,
+  walkthrough for avatar drag and release, walls, key, vision, compass,
   exit, and meeting the other player. Each step uses a large icon; short
   Russian text is secondary support for an adult.
 - The walkthrough provides a replay button and an explicit Russian skip button