feat(email): PWA login sends code only; drop admin link from alert emails

Two email changes, per the owner:

1. Code-only login from an installed PWA. A login requested while running as a
   standalone PWA now omits the one-tap confirm link from the email — the link
   would open in a separate browser whose minted session cannot reach the PWA,
   stranding the login. The code is typed in the same window instead. The client
   sends a `pwa` flag (isStandalone) on the email-code request (a new FBS field,
   threaded through the gateway); the backend omits the deeplink when it is set,
   reusing the existing deletion-code link-omission path. Non-PWA browser logins
   keep the one-tap link. No polling, no migration.

2. Security: the operator alert digest no longer embeds the admin-console (/_gm)
   URL. An admin link must never travel in an email, where a mail provider could
   cache or index it; the operator opens the console directly.

Tests: an inttest asserting the PWA login email omits the link (and a browser one
keeps it); a codec round-trip of the new pwa field; the alert-digest test flipped
to guard the admin link is absent. Docs: ARCHITECTURE + FUNCTIONAL (+ru).

ui/src/lib/client.ts

@@ -65,7 +65,7 @@ export interface GatewayClient {
    *  cosmetic seed for a brand-new account). */
   authVK(params: string, displayName: string): Promise<Session>;
   authGuest(locale?: string): Promise<Session>;
-  authEmailRequest(email: string, language: string): Promise<void>;
+  authEmailRequest(email: string, language: string, pwa: boolean): Promise<void>;
   authEmailLogin(email: string, code: string): Promise<Session>;
   /** Confirm a one-tap email deeplink token: a login returns a session to adopt; a
    *  link returns a status ('confirmed' | 'merge_required'). */