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/chat/Message.tsx

@@ -286,7 +286,7 @@ export function Message({ message, isCompact, isFirstInGroup }: MessageProps) {
                       key={att.id}
                       href={`/api/uploads/${att.filename}`}
                       download={att.originalName}
-                      className="flex items-center gap-3 p-4 bg-[rgba(26,26,35,0.50)] rounded-lg border border-border-hard hover:bg-interactive-hover transition-all max-w-[400px] mt-1 group/att"
+                      className="flex items-center gap-3 p-4 bg-surface-channel/50 rounded-lg border border-border-hard hover:bg-interactive-hover transition-all max-w-[400px] mt-1 group/att"
                     >
                       <div className="p-2 bg-surface-base rounded text-txt-tertiary group-hover/att:text-txt-primary transition-colors">
                         <svg className="w-8 h-8" fill="none" viewBox="0 0 24 24" stroke="currentColor">