feat: stateless federated identity resolver + federation UX improvements

Add identity.ts with isSelf() and resolveDisplayIdentity() — pure
stateless functions that detect replicated-self using the immutable
(username, homeInstance) composite key. No store lookups, no data
mutation. Fixes wrong avatar gradient and missing edit/delete on
own messages in remote channels.

Also includes: optimistic message dedup fix for cross-instance
messages (content-only matching), federation toast notifications,
Username component with @domain display, invite parser, and
deploy script simplification.

packages/web/src/components/ui/Username.tsx

@@ -0,0 +1,25 @@
+import React from 'react';
+import { Tooltip } from './Tooltip';
+
+interface UsernameProps {
+  username: string;
+  className?: string;
+  style?: React.CSSProperties;
+}
+
+export function Username({ username, className, style }: UsernameProps) {
+  const atIndex = username.indexOf('@');
+  if (atIndex === -1) {
+    return <span className={className} style={style}>{username}</span>;
+  }
+  const name = username.slice(0, atIndex);
+  const domain = username.slice(atIndex + 1);
+  return (
+    <Tooltip content={username} position="top">
+      <span className={className} style={style}>
+        {name}
+        <span className="text-txt-tertiary text-[0.8em] ml-0.5 font-normal">@{domain}</span>
+      </span>
+    </Tooltip>
+  );
+}