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/base.html

@@ -122,7 +122,7 @@
                         <span class="text-sm opacity-80">{{ _('Language') }}:</span>
                         {% for lang_code, lang_name in config['LANGUAGES'].items() %}
                             <a 
-                                href="?lang={{ lang_code }}"
+                                href="{{ url_for('set_language', lang=lang_code) }}"
                                 class="px-2 py-1 text-sm rounded hover:bg-white/20 transition-colors
                                        {% if language == lang_code %}bg-white/30 font-medium{% endif %}"
                             >