Allow text selection in the interface

Remove user-select: none from body, keeping it only on interactive
elements (buttons, sliders, tooltips) where it prevents accidental
selection during interaction.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

bicorder-app/src/app.css

@@ -45,7 +45,6 @@ body {
   color: var(--fg-color);
   min-height: 100vh;
   touch-action: manipulation;
-  user-select: none;
   -webkit-tap-highlight-color: transparent;
   font-size: 16px;
 }