Add seed/Telegram login button and traffic usage indicator

New non-blocking login panel on the Home screen (seed backup/restore +
Telegram magic-link via the bot deep-link, polled through the new backend
endpoints) — uses its own popover instead of a full-screen Dialog so the
burger menu stays clickable while it's open. VpnStats now shows a used/limit
usage bar backed by a new useUsageStore.

Also registers tauri-plugin-shell (needed to open the t.me deep-link) and
threads an optional auth token from the app down through TunnelConfig to the
proxy client (desktop + Android), for instances running --require-auth.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/pages/Home.tsx

@@ -1,10 +1,12 @@
 import { VpnControl } from "../features/VpnControl";
 import { VpnStats } from "../features/VpnStats";
 import { CountrySelect } from "../features/CountrySelect";
+import { LoginButton } from "../features/LoginButton";
 
 export function Home() {
   return (
     <div className="relative flex flex-col gap-6 items-center justify-center w-full h-full">
+      <LoginButton />
       <VpnControl />
       <CountrySelect />
       <VpnStats />