When generating an offer from the recruitment application and signing it, the applicant's email address is incorrectly displayed.

### **Steps to Reproduce:**
1) Install sign, recruitment, hr_contract_salary
2) Create an new application and add basic detail like name and email as
   (path and `path@test.com`)
3) Generate offer and sign with all the required signer(applicant and Marc Demo). 
4) Open the application form view and open the certificate.

### **Observed Behavior:**
Email is not set correctly in the generated certificate (appearing as `john@example.com`).

### **Expected Behavior:**
The email of the applicant should be correctly set(e.g as `path@test.com`)

### **Root Cause:**
When the applicant signs the document, their email is explicitly set to `False` at [1]. This is done because the applicant is not linked to any user yet. Later, when generating the certificate, the system attempts to display the user's partner email at [2], which is `False`, causing the default fallback value (`john@example.com`) to be printed.

[1]- https://github.com/odoo/enterprise/blob/49226f4109c7d7bb48340949951e70f4245d0e5b/hr_contract_salary/controllers/main.py#L53-L54

[2]- https://github.com/odoo/enterprise/blob/49226f4109c7d7bb48340949951e70f4245d0e5b/sign/report/sign_log_reports.xml#L59

### **Fix:**
Use `signer_email` instead of the partner's email to ensure the correct email is displayed on the certificate every time.

opw-6280170

Forward-Port-Of: odoo/enterprise#120566