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.

gateway/internal/backendclient/api_social.go

@@ -335,6 +335,31 @@ func (c *Client) LinkTelegramMerge(ctx context.Context, userID, externalID strin
 	return out, err
 }
 
+// ChangeEmailRequest asks the backend to mail a confirm-code to a new address for an
+// authenticated email change.
+func (c *Client) ChangeEmailRequest(ctx context.Context, userID, email string) error {
+	return c.do(ctx, http.MethodPost, "/api/v1/user/link/email/change/request", userID, "",
+		map[string]string{"email": email}, nil)
+}
+
+// ChangeEmailConfirm verifies the code and atomically switches the account's email,
+// returning the refreshed profile in the result.
+func (c *Client) ChangeEmailConfirm(ctx context.Context, userID, email, code string) (LinkResultResp, error) {
+	var out LinkResultResp
+	err := c.do(ctx, http.MethodPost, "/api/v1/user/link/email/change/confirm", userID, "",
+		map[string]string{"email": email, "code": code}, &out)
+	return out, err
+}
+
+// LinkUnlink detaches a platform identity (kind = "telegram" | "vk") from the caller
+// and returns the refreshed profile in the result.
+func (c *Client) LinkUnlink(ctx context.Context, userID, kind string) (LinkResultResp, error) {
+	var out LinkResultResp
+	err := c.do(ctx, http.MethodPost, "/api/v1/user/link/unlink", userID, "",
+		map[string]string{"kind": kind}, &out)
+	return out, err
+}
+
 // Stats returns the caller's lifetime statistics.
 func (c *Client) Stats(ctx context.Context, userID string) (StatsResp, error) {
 	var out StatsResp