[Web] Add show/hide password toggle on login pages

Adds an eye icon button to the password field on the user, admin, and
domain admin login pages. Clicking it toggles between hidden and visible
password text, using Bootstrap Icons (bi-eye / bi-eye-slash).

Closes #6893

data/web/templates/admin_index.twig

@@ -49,6 +49,7 @@
             <div class="input-group">
               <div class="input-group-text"><i class="bi bi-lock-fill"></i></div>
               <input name="pass_user" type="password" id="pass_user" class="form-control" placeholder="{{ lang.login.password }}" required="" autocomplete="current-password">
+              <button type="button" class="btn btn-outline-secondary" onclick="var i=document.getElementById('pass_user'),ic=this.querySelector('i');if(i.type==='password'){i.type='text';ic.className='bi bi-eye-slash';}else{i.type='password';ic.className='bi bi-eye';}" tabindex="-1" aria-label="Toggle password visibility"><i class="bi bi-eye"></i></button>
             </div>
           </div>
           <div class="d-flex justify-content-between mt-4" style="position: relative">

data/web/templates/domainadmin_index.twig

@@ -49,6 +49,7 @@
             <div class="input-group">
               <div class="input-group-text"><i class="bi bi-lock-fill"></i></div>
               <input name="pass_user" type="password" id="pass_user" class="form-control" placeholder="{{ lang.login.password }}" required="" autocomplete="current-password">
+              <button type="button" class="btn btn-outline-secondary" onclick="var i=document.getElementById('pass_user'),ic=this.querySelector('i');if(i.type==='password'){i.type='text';ic.className='bi bi-eye-slash';}else{i.type='password';ic.className='bi bi-eye';}" tabindex="-1" aria-label="Toggle password visibility"><i class="bi bi-eye"></i></button>
             </div>
           </div>
           <div class="d-flex justify-content-between mt-4" style="position: relative">

data/web/templates/user_index.twig

@@ -56,6 +56,7 @@
             <div class="input-group">
               <div class="input-group-text"><i class="bi bi-lock-fill"></i></div>
               <input name="pass_user" type="password" id="pass_user" class="form-control" placeholder="{{ lang.login.password }}" required="" autocomplete="current-password">
+              <button type="button" class="btn btn-outline-secondary" onclick="var i=document.getElementById('pass_user'),ic=this.querySelector('i');if(i.type==='password'){i.type='text';ic.className='bi bi-eye-slash';}else{i.type='password';ic.className='bi bi-eye';}" tabindex="-1" aria-label="Toggle password visibility"><i class="bi bi-eye"></i></button>
             </div>
           </div>
           <div class="mt-2 text-muted" style="font-size: 0.9rem;">