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/add_user.html

@@ -1,30 +1,30 @@
 {% extends "base.html" %}
 
 {% block content %}
-    <h2>Add New User</h2>
+    <h2>{{ _('Add New User') }}</h2>
     <form method="POST" action="/add_user">
         <div class="form-group">
-            <label for="name">Name:</label>
+            <label for="name">{{ _('Name') }}:</label>
             <input type="text" id="name" name="name" required>
         </div>
 
         <div class="form-group">
-            <label for="email">Email:</label>
+            <label for="email">{{ _('Email') }}:</label>
             <input type="email" id="email" name="email">
         </div>
 
         <div class="form-group">
-            <label for="phone">Phone:</label>
+            <label for="phone">{{ _('Phone') }}:</label>
             <input type="text" id="phone" name="phone">
         </div>
 
         <div class="form-group">
-            <label for="identification">Identification:</label>
+            <label for="identification">{{ _('Identification') }}:</label>
             <input type="text" id="identification" name="identification" required>
         </div>
 
         <div class="form-group">
-            <button type="submit" class="btn">Add User</button>
+            <button type="submit" class="btn">{{ _('Add User') }}</button>
         </div>
     </form>
-{% endblock %}
+{% endblock %}