feat(i18n): add Spanish localization support with Flask-Babel

- Add Flask-Babel for internationalization
- Configure app for Spanish (default) and English
- Add language switcher in navigation
- Wrap all UI text in templates with gettext()
- Create translation directory structure
- Add complete Spanish translations (58 strings)
- Add English translations (template)
- Compile .po to .mo files
- Add extract_translations.py script for future updates
- Add compile_translations.py script

Spanish is now the default language, with English available via ?lang=en

Translated strings include:
- Navigation (Home, Add Tablet, Add User, etc.)
- User Loans interface (Search, Filter, Status, etc.)
- Table headers (User, Tablet, Serial Number, etc.)
- Buttons (Loan, Return, Search, etc.)
- Messages (No users found, No active loans, etc.)
- All UI text across all templates

app.py

@@ -8,10 +8,46 @@ from flask import Flask, render_template, request, redirect, url_for, flash
 import sqlite3
 import os
 from datetime import datetime
+from flask_babel import Babel, gettext, lazy_gettext
 
 app = Flask(__name__)
 app.secret_key = 'your_secret_key_here'
 
+# Babel configuration
+babel = Babel(app)
+
+# Configure supported languages
+app.config['BABEL_DEFAULT_LOCALE'] = 'es'
+app.config['LANGUAGES'] = {
+    'en': 'English',
+    'es': 'Español'
+}
+
+# Configure translation directory
+app.config['BABEL_TRANSLATION_DIRECTORIES'] = 'translations'
+
+@babel.localeselector
+def get_locale():
+    # Try to get language from URL parameter
+    lang = request.args.get('lang')
+    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())
+
+# Context processor to make language and config available in templates
+@app.context_processor
+def inject_global_variables():
+    from flask import request
+    lang = get_locale()
+    return {
+        'language': lang,
+        'config': app.config
+    }
+
 # Database setup
 DATABASE = 'tablets.db'