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/auth/LoginPage.tsx

@@ -42,7 +42,7 @@ export function LoginPage() {
 
         <form onSubmit={handleSubmit}>
           {error && (
-            <div className="mb-4 p-3 bg-[rgba(253,164,175,0.10)] border border-[rgba(253,164,175,0.30)] rounded text-txt-danger text-sm">
+            <div className="mb-4 p-3 bg-accent-rose/10 border border-accent-rose/30 rounded text-txt-danger text-sm">
               {error}
             </div>
           )}

packages/web/src/components/auth/RegisterPage.tsx

@@ -54,7 +54,7 @@ export function RegisterPage() {
 
         <form onSubmit={handleSubmit}>
           {error && (
-            <div className="mb-4 p-3 bg-[rgba(253,164,175,0.10)] border border-[rgba(253,164,175,0.30)] rounded text-txt-danger text-sm">
+            <div className="mb-4 p-3 bg-accent-rose/10 border border-accent-rose/30 rounded text-txt-danger text-sm">
               {error}
             </div>
           )}