refactor: convert color system to RGB channels for native Tailwind opacity

Root cause fix for all broken CSS variable opacity modifiers. Converted
every hex CSS variable to space-separated RGB channels (e.g. #fda4af →
253 164 175) and updated Tailwind config to use rgb(var(...) / <alpha-value>).
This makes bg-accent-rose/10, border-border-hard/50, etc. work natively
everywhere — no more currentColor fallback. Reverted all previous explicit
rgba() workarounds back to clean Tailwind syntax. Also fixed the voice
connection wifi icon SVG (was bottom-left aligned, now centered).

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

@@ -134,7 +134,7 @@ export function VoiceGrid({ participants }: VoiceGridProps) {
 
         {/* Bottom strip of other tiles */}
         {!stripHidden && otherTiles.length > 0 && (
-          <div className="h-[120px] flex-shrink-0 flex items-center justify-center gap-2 p-2 bg-[rgba(11,11,16,0.50)] overflow-x-auto no-scrollbar">
+          <div className="h-[120px] flex-shrink-0 flex items-center justify-center gap-2 p-2 bg-surface-base/50 overflow-x-auto no-scrollbar">
             {otherTiles.map((t) => (
               <div
                 key={t.key}