fix: prevent native text selection on mobile long-press

iOS triggered both the app context menu and native text selection overlay
on long-press. Three-layer fix: CSS user-select:none on buttons/[role=button]/
[data-context-menu] elements; non-passive touchstart listener with conditional
preventDefault on context-menu targets; removeAllRanges() at timer fire to
clear any residual selection. Added data-context-menu to all mobile surfaces
that use onContextMenu handlers.

packages/web/src/components/layout/MobileDmsScreen.tsx

@@ -165,6 +165,7 @@ export function MobileDmsScreen() {
           return (
             <button
               key={dm.id}
+              data-context-menu
               onClick={() => handleDmTap(dm.id)}
               onContextMenu={(e) => handleDmContextMenu(e, dm.id, isGroup)}
               className="w-full flex items-center gap-3 px-4 py-3 hover:bg-interactive-hover text-left transition-colors"