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).

pkg/fbs/scrabble.fbs

@@ -138,6 +138,11 @@ table EmailRequestRequest {
   email:string;
   browser_tz:string;
   language:string;
+  // Set when the request originates from an installed PWA (standalone display mode): the
+  // backend then omits the one-tap confirm link from the login email so the code is typed in
+  // the same window, avoiding a link that opens in a separate browser (a different storage
+  // context) where the minted session could not reach the PWA.
+  pwa:bool;
 }
 
 // EmailLoginRequest logs in to the account owning email (provisioned at the