feat: unified glass material system for all floating surfaces

Add .glass-modal CSS class (0.82 opacity) for dialogs and apply
consistent glass tiers across all floating UI: modals, context menus,
tooltips, popovers, and call cards. Replaces ad-hoc bg-surface-elevated
and inline styles with the documented 5-tier glass hierarchy.

packages/web/src/components/voice/IncomingCallModal.tsx

@@ -51,10 +51,10 @@ export function IncomingCallModal() {
   return (
     <div className="fixed inset-0 z-[100] flex items-center justify-center">
       {/* Backdrop */}
-      <div className="absolute inset-0 bg-black/60" />
+      <div className="absolute inset-0 bg-black/50" />
 
       {/* Call card */}
-      <div className="relative bg-surface-channel rounded-lg shadow-2xl w-[340px] overflow-hidden">
+      <div className="relative glass-modal rounded-lg w-[340px] overflow-hidden">
         {/* Ring animation background */}
         <div className="absolute inset-0 overflow-hidden">
           <div className="absolute top-1/2 left-1/2 -translate-x-1/2 -translate-y-1/2 w-[200px] h-[200px] rounded-full bg-status-online/5 animate-ping" style={{ animationDuration: '2s' }} />

packages/web/src/components/voice/VoiceUserContextMenu.tsx

@@ -192,7 +192,7 @@ function MoveToSubmenu({ channels, onMove, btnClass, btnStyle }: MoveToSubmenuPr
       {open && ReactDOM.createPortal(
         <div
           ref={flyoutRef}
-          className="fixed z-[210] bg-surface-elevated rounded-md shadow-elevation-high py-1.5 min-w-[160px] overflow-y-auto scrollbar-thin animate-fade-in"
+          className="fixed z-[210] glass rounded-md py-1.5 min-w-[160px] overflow-y-auto scrollbar-thin animate-fade-in"
           style={{ left: -9999, top: -9999 }}
           onMouseEnter={cancelCloseTimer}
           onMouseLeave={startCloseTimer}
@@ -291,7 +291,7 @@ export function VoiceUserContextMenu({ targetUserId, channelId, position, onClos
   return ReactDOM.createPortal(
     <div
       ref={menuRef}
-      className="fixed z-[200] bg-surface-elevated rounded-md shadow-elevation-high min-w-[200px] max-h-[calc(100vh-16px)] overflow-y-auto scrollbar-thin animate-fade-in"
+      className="fixed z-[200] glass rounded-md min-w-[200px] max-h-[calc(100vh-16px)] overflow-y-auto scrollbar-thin animate-fade-in"
       style={{ left: position.x, top: position.y }}
       onMouseDown={(e) => e.stopPropagation()}
       onClick={(e) => e.stopPropagation()}