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

app.py

@@ -4,7 +4,7 @@ Simple Tablet Lending and Return Management Web Application
 with SQLite backend
 """
 
-from flask import Flask, render_template, request, redirect, url_for, flash
+from flask import Flask, render_template, request, redirect, url_for, flash, make_response, session
 import sqlite3
 import os
 from datetime import datetime
@@ -25,15 +25,41 @@ app.config['BABEL_TRANSLATION_DIRECTORIES'] = 'translations'
 
 # Locale selector function
 def get_locale():
-    # Try to get language from URL parameter
-    lang = request.args.get('lang')
+    # Try to get language from cookie
+    lang = request.cookies.get('language')
     if lang and lang in app.config['LANGUAGES']:
         return lang
     # Try to get from session
     if hasattr(request, 'session') and 'language' in request.session:
         return request.session['language']
-    # Try to get from browser preferences
-    return request.accept_languages.best_match(app.config['LANGUAGES'].keys())
+    # Default to Spanish (es) for internal app
+    return 'es'
+
+
+# Before request handler to set language from URL param and persist to cookie/session
+@app.before_request
+def handle_language():
+    lang = request.args.get('lang')
+    if lang and lang in app.config['LANGUAGES']:
+        # Store in session
+        session['language'] = lang
+        # Set cookie for longer persistence (1 year)
+        # Note: We need to use response.set_cookie, but before_request can't modify response
+        # So we'll handle this in a separate route
+    return None
+
+
+# Language switcher endpoint
+@app.route('/set_language')
+def set_language():
+    lang = request.args.get('lang')
+    if lang and lang in app.config['LANGUAGES']:
+        session['language'] = lang
+        # Set cookie
+        resp = make_response(redirect(request.referrer or '/'))
+        resp.set_cookie('language', lang, max_age=31536000)  # 1 year
+        return resp
+    return redirect(request.referrer or '/')
 
 # Initialize Babel with locale selector
 babel = Babel(app, locale_selector=get_locale)