feat(account): unlink provider + change-email edges (backend + gateway)

Unlink: POST /user/link/unlink (telegram|vk) via account.RemoveIdentity, refusing
the last identity; email is never unlinked. New fbs LinkUnlinkRequest + gateway
link.unlink op, returning the refreshed profile.

Change-email: purposeChange confirm-codes (RequestChangeCode/ConfirmChange) that
atomically replace the account's confirmed email (account.replaceEmailIdentity);
a new address owned by another account is refused without disclosure, never merged.
The one-tap deeplink handles purposeChange too. Reuses the LinkEmail* fbs tables;
gateway link.email.change.{request,confirm} ops + backendclient methods; branded
ru/en change-email copy.

backend/internal/account/emailtemplate.go

@@ -80,6 +80,24 @@ var confirmEmailCopy = map[string]map[string]emailCopy{
 			FooterIgnore: "Если вы не запрашивали это письмо, просто проигнорируйте его.",
 		},
 	},
+	purposeChange: {
+		"en": {
+			Subject:      "Confirm your new Erudit e-mail",
+			Preheader:    "Confirm your new address",
+			Heading:      "Confirm your new e-mail",
+			Intro:        "Enter this code to switch your account to this address:",
+			CTALabel:     "Confirm with one tap",
+			FooterIgnore: "If you didn't request this change, you can safely ignore it — your address stays the same.",
+		},
+		"ru": {
+			Subject:      "Подтвердите новый e-mail в Эрудит",
+			Preheader:    "Подтвердите новый адрес",
+			Heading:      "Смена e-mail",
+			Intro:        "Введите этот код, чтобы привязать аккаунт к новому адресу:",
+			CTALabel:     "Подтвердить одним нажатием",
+			FooterIgnore: "Если вы не запрашивали смену, просто проигнорируйте письмо — адрес останется прежним.",
+		},
+	},
 }
 
 // emailBrand is the brand wordmark per locale.