fix(bitmap viewer): toolbar above the touch joystick zones on mobile

The fullscreen / PFP / exit toggles in .bitmap-toolbar sat at default
stacking — they were hidden behind the renderer's right joystick zone
(z-index 2) and jump button (z-index 3) on mobile, which both cover
the bottom-right quadrant. Promote the toolbar to z-index 10 so it
clears both.

Author: hans-crypto

frontend/src/app/components/_ordpool/digital-artifact-viewer/bitmap-viewer/bitmap-viewer.component.scss

@@ -25,6 +25,12 @@
   bottom: 10px;
   display: flex;
   gap: 8px;
+  // Sit above the renderer's touch UI. The renderer's right joystick
+  // zone (z-index 2) covers the bottom-right quadrant and the jump
+  // button (z-index 3) sits on top of that; without an explicit
+  // z-index here the toolbar's fullscreen / PFP / exit toggles would
+  // be hidden under both on mobile.
+  z-index: 10;
 }
 
 .bitmap-toggle {