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

translations/messages.pot

@@ -0,0 +1,195 @@
+msgid ""
+msgstr ""
+"Project-Id-Version: Tablet Management System
+"
+"POT-Creation-Date: 2026-06-20
+"
+"PO-Revision-Date: YEAR-MO-DA HO:MI+ZONE
+"
+"Last-Translator: 
+"
+"Language-Team: 
+"
+"Language: en
+"
+"MIME-Version: 1.0
+"
+"Content-Type: text/plain; charset=UTF-8
+"
+"Content-Transfer-Encoding: 8bit
+"
+
+msgid "Actions"
+msgstr ""
+
+msgid "Active Loans"
+msgstr ""
+
+msgid "Add Tablet"
+msgstr ""
+
+msgid "Add User"
+msgstr ""
+
+msgid "All Users"
+msgstr ""
+
+msgid "All users have at least one active loan"
+msgstr ""
+
+msgid "Available Tablets"
+msgstr ""
+
+msgid "Back to All Users"
+msgstr ""
+
+msgid "Borrower"
+msgstr ""
+
+msgid "Brand"
+msgstr ""
+
+msgid "Call"
+msgstr ""
+
+msgid "Clear Filters"
+msgstr ""
+
+msgid "Email"
+msgstr ""
+
+msgid "History"
+msgstr ""
+
+msgid "Home"
+msgstr ""
+
+msgid "ID"
+msgstr ""
+
+msgid "Identification"
+msgstr ""
+
+msgid "Language"
+msgstr ""
+
+msgid "Last Loan Date"
+msgstr ""
+
+msgid "Loan"
+msgstr ""
+
+msgid "Loan Date"
+msgstr ""
+
+msgid "Loan History"
+msgstr ""
+
+msgid "Loan Status"
+msgstr ""
+
+msgid "Loan Tablet"
+msgstr ""
+
+msgid "Model"
+msgstr ""
+
+msgid "Next"
+msgstr ""
+
+msgid "No Active Loans"
+msgstr ""
+
+msgid "No active loans"
+msgstr ""
+
+msgid "No available tablets"
+msgstr ""
+
+msgid "No loan history for this user"
+msgstr ""
+
+msgid "No users found"
+msgstr ""
+
+msgid "No users have active loans"
+msgstr ""
+
+msgid "No users match your search for"
+msgstr ""
+
+msgid "Non-Loanable Devices"
+msgstr ""
+
+msgid "Not returned"
+msgstr ""
+
+msgid "Notes"
+msgstr ""
+
+msgid "Previous"
+msgstr ""
+
+msgid "Project Management"
+msgstr ""
+
+msgid "Return"
+msgstr ""
+
+msgid "Return Date"
+msgstr ""
+
+msgid "Returned"
+msgstr ""
+
+msgid "Search"
+msgstr ""
+
+msgid "Search Users"
+msgstr ""
+
+msgid "Search by name or identification..."
+msgstr ""
+
+msgid "Serial Number"
+msgstr ""
+
+msgid "Showing"
+msgstr ""
+
+msgid "Status"
+msgstr ""
+
+msgid "Tablet"
+msgstr ""
+
+msgid "Tablet Management System"
+msgstr ""
+
+msgid "Tablet Management System - Internal Tool"
+msgstr ""
+
+msgid "There are no users in the system yet"
+msgstr ""
+
+msgid "User"
+msgstr ""
+
+msgid "User Loans"
+msgstr ""
+
+msgid "View Details"
+msgstr ""
+
+msgid "With Active Loans"
+msgstr ""
+
+msgid "of"
+msgstr ""
+
+msgid "to"
+msgstr ""
+
+msgid "users"
+msgstr ""
+