fix(i18n): enable translations in all templates and implement cookie-based language persistence

- Add _() translation function to add_tablet.html, add_user.html, loan_tablet.html
- Add _() translation function to add_non_loanable_device.html, edit_non_loanable_device.html
- Add _() translation function to history.html, non_loanable_devices.html, project_management.html
- Update app.py to use cookie-based language persistence with 1-year expiry
- Add /set_language endpoint to handle language switching with cookie + session
- Update language switcher in base.html to use new endpoint
- Set Spanish (es) as default language for internal app
- Import make_response and session from Flask

templates/history.html

@@ -2,18 +2,18 @@
 
 {% block content %}
     <div class="section">
-        <h2>Loan History</h2>
+        <h2>{{ _('Loan History') }}</h2>
         {% if loans %}
             <div class="table-container">
                 <table>
                     <thead>
                         <tr>
-                            <th>Tablet</th>
-                            <th>Serial Number</th>
-                            <th>Borrower</th>
-                            <th>Loan Date</th>
-                            <th>Return Date</th>
-                            <th>Status</th>
+                            <th>{{ _('Tablet') }}</th>
+                            <th>{{ _('Serial Number') }}</th>
+                            <th>{{ _('Borrower') }}</th>
+                            <th>{{ _('Loan Date') }}</th>
+                            <th>{{ _('Return Date') }}</th>
+                            <th>{{ _('Status') }}</th>
                         </tr>
                     </thead>
                     <tbody>
@@ -31,7 +31,7 @@
                 </table>
             </div>
         {% else %}
-            <p>No loan history available.</p>
+            <p>{{ _('No loan history available.') }}</p>
         {% endif %}
     </div>
-{% endblock %}
+{% endblock %}