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/chat/MentionPopover.tsx

@@ -58,7 +58,7 @@ export function MentionPopover({ query, selectedIndex, onSelect, anchorRef }: Me
 
   return createPortal(
     <div ref={floatingRef} style={style} className="w-[280px]">
-      <div className="bg-surface-elevated rounded-lg shadow-[0_0_0_1px_rgba(0,0,0,0.15),0_8px_16px_rgba(0,0,0,0.24)] overflow-hidden max-h-[320px] overflow-y-auto scrollbar-thin">
+      <div className="glass rounded-lg overflow-hidden max-h-[320px] overflow-y-auto scrollbar-thin">
         <div className="px-2 py-1.5 text-[11px] font-bold text-txt-tertiary uppercase tracking-wider">
           Members
         </div>