fix(ui): remove backdrop-filter to fix WebKitGTK blank screen

WebKitGTK on some GPU/driver configs fails to composite windows that use
backdrop-filter blur, causing the entire window to render blank. Replace all
glassmorphism surfaces with solid dark backgrounds (#0a0a0f / #12121a).

ui/stream/src/components/BeamBar.tsx

@@ -22,8 +22,7 @@ export function BeamBar({
       className="fixed bottom-0 left-0 right-0 z-[100] flex items-center gap-3 px-6 border-t border-white/[0.08]"
       style={{
         height: 72,
-        background: 'rgba(10,10,15,0.95)',
-        backdropFilter: 'blur(16px)',
+        background: '#0a0a0f',
       }}
     >
       <BeamAvatar size={32} state={beamState} />

ui/stream/src/components/BeamMessageOverlay.tsx

@@ -12,7 +12,7 @@ export function BeamMessageOverlay({ messages, onClear }: BeamMessageOverlayProp
 
   return (
     <div className="fixed top-16 right-6 z-[90] w-80 max-w-[90vw]">
-      <div className="rounded-xl border border-white/10 bg-[#12121a]/95 backdrop-blur-xl p-4 shadow-2xl">
+      <div className="rounded-xl border border-white/10 bg-[#12121a] p-4 shadow-2xl">
         <div className="flex items-center justify-between mb-3">
           <span className="text-xs font-semibold text-blue-400 uppercase tracking-wider">
             Beam

ui/stream/src/components/ChannelHeader.tsx

@@ -31,8 +31,7 @@ export function ChannelHeader({
     <div
       className="sticky top-0 z-10 flex items-center justify-between px-6 py-3 border-b border-white/[0.04]"
       style={{
-        background: 'rgba(10,10,15,0.9)',
-        backdropFilter: 'blur(8px)',
+        background: '#0a0a0f',
       }}
     >
       <div className="flex items-center gap-3">

ui/stream/src/components/ExpandedMenu.tsx

@@ -76,8 +76,7 @@ export function ExpandedMenu({
       <div
         className="fixed left-0 top-0 bottom-0 z-[60] w-[280px] border-r border-white/[0.06] flex flex-col"
         style={{
-          background: 'rgba(10,10,15,0.98)',
-          backdropFilter: 'blur(20px)',
+          background: '#0a0a0f',
         }}
       >
         <div className="flex items-center justify-between p-4 border-b border-white/[0.06]">

ui/stream/src/components/TopBar.tsx

@@ -23,8 +23,7 @@ export function TopBar({ onMenuToggle, profile }: TopBarProps) {
       className="fixed top-0 left-0 right-0 z-50 flex items-center justify-between px-4 border-b border-white/[0.06]"
       style={{
         height: 48,
-        background: 'rgba(10,10,15,0.85)',
-        backdropFilter: 'blur(12px)',
+        background: '#0a0a0f',
       }}
     >
       <div className="flex items-center gap-3">