fix(desktop): crisp terminal text via opaque xterm canvas

The terminal looked soft/heavy on every platform because the xterm
Terminal was built with allowTransparency: true, which drops the WebGL
renderer's opaque fast-path and bakes glyphs as grayscale-alpha coverage
for compositing over a see-through canvas. Our surface (--ui-bg-chrome)
is opaque and withSurface already paints it, so transparency was pure
blur for no benefit — VS Code keeps it off too. Also drop the Medium
(500) base weight for normal/bold (400/700) to match VS Code's metrics,
and remove the now-unused JetBrains Mono Medium face + woff2.

apps/desktop/src/styles.css

@@ -26,13 +26,6 @@
   font-display: swap;
   src: url('./fonts/JetBrainsMono-Regular.woff2') format('woff2');
 }
-@font-face {
-  font-family: 'JetBrains Mono';
-  font-style: normal;
-  font-weight: 500;
-  font-display: swap;
-  src: url('./fonts/JetBrainsMono-Medium.woff2') format('woff2');
-}
 @font-face {
   font-family: 'JetBrains Mono';
   font-style: normal;