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"

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

@@ -94,7 +94,7 @@ export function MobileFolderSheet({ folder, onClose, onSelectSpace, onUpdateFold
         <div className="w-10 h-1 bg-txt-tertiary/30 rounded-full mx-auto mt-2 mb-1 shrink-0" />
 
         {/* Folder header */}
-        <div className="px-4 py-2 flex items-center gap-2 shrink-0" onContextMenu={handleFolderContextMenu}>
+        <div className="px-4 py-2 flex items-center gap-2 shrink-0" data-context-menu onContextMenu={handleFolderContextMenu}>
           <div
             className="w-5 h-5 rounded"
             style={{ background: folder.color || 'rgb(var(--text-tertiary))' }}

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

@@ -669,6 +669,7 @@ export function MobileSpacesScreen() {
                 />
               </div>
               <button
+                data-context-menu
                 onClick={() => handleSpaceSelect(space.id)}
                 onContextMenu={(e) => handleSpaceContextMenu(e, space.id)}
                 className={`w-10 h-10 rounded-2xl overflow-hidden flex items-center justify-center transition-all ${
@@ -765,6 +766,7 @@ export function MobileSpacesScreen() {
             return (
               <div key={category.id} className="mt-3">
                 <button
+                  data-context-menu
                   onClick={() => toggleCategory(category.id)}
                   onContextMenu={(e) => handleCategoryContextMenu(e, category.id)}
                   className="flex items-center gap-1 px-1 py-1 w-full text-left"

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

@@ -198,6 +198,7 @@ export function MobileVoiceFullScreen() {
             return (
               <div
                 key={userId}
+                data-context-menu
                 className={`rounded-xl bg-surface-channel p-4 flex flex-col items-center gap-3 ${
                   participantIds.length <= 2 ? 'py-8' : 'py-4'
                 }`}