fix(auth): address second-round review findings

- OAuth-cancel feedback now keys on the live session, not link_uid: ANY
  signed-in cancel (add-another-account via /auth/login, link, or upgrade)
  redirects to /?oauth=cancelled and gets a neutral "Google sign-in was
  cancelled" toast — the add-account path set no marker and was silent, and
  the shared ?link= message wrongly called an upgrade "account linking"
- drop 127.0.0.1 from the dev OAuth host allowlist: only the localhost
  callbacks are registered in the Google console, so a 127.0.0.1 origin now
  falls back to the fixed callback instead of a redirect_uri_mismatch
- Welcome: extract the shared goResend handler (banner CTA + sign-in link) and
  merge the two adjacent signin-only blocks

frontend/src/i18n/locales/en/settings.json

@@ -78,9 +78,9 @@
       "linked": "Google account linked.",
       "conflict": "That Google account is already linked to a different Siftlode account.",
       "mismatch": "That's a different Google account than the one already linked here. Sign in with the linked one.",
-      "error": "Couldn't link the Google account. Please try again.",
-      "cancelled": "Google account linking was cancelled."
+      "error": "Couldn't link the Google account. Please try again."
     },
+    "googleCancelled": "Google sign-in was cancelled.",
     "password": {
       "title": "Password",
       "setHint": "A password is set. You can sign in with your email and password.",

frontend/src/i18n/locales/hu/settings.json

@@ -78,9 +78,9 @@
       "linked": "Google-fiók összekapcsolva.",
       "conflict": "Ez a Google-fiók már egy másik Siftlode-fiókhoz van kapcsolva.",
       "mismatch": "Ez nem az a Google-fiók, amely ide már össze van kapcsolva. Az összekapcsolttal jelentkezz be.",
-      "error": "Nem sikerült összekapcsolni a Google-fiókot. Próbáld újra.",
-      "cancelled": "A Google-fiók összekapcsolása megszakadt."
+      "error": "Nem sikerült összekapcsolni a Google-fiókot. Próbáld újra."
     },
+    "googleCancelled": "A Google-belépés megszakadt.",
     "password": {
       "title": "Jelszó",
       "setHint": "Van beállított jelszó. Bejelentkezhetsz e-mail-címmel és jelszóval.",