feat: add next-intl i18n, Spanish translation, language selector, glossary tooltips

[Adeolu01]

Summary

• Installs next-intl and configures it via next.config.js using createNextIntlPlugin
• i18n.ts provides getRequestConfig with locale validation and dynamic message loading
• middleware.ts runs locale detection from the Accept-Language header, falls back to en, and enables URL-prefix routing (/es/) via localePrefix: "as-needed" so existing routes at /, /connect, /escrow, etc. are unaffected
• messages/en.json and messages/es.json cover navigation labels, hero section, form fields, error messages, escrow status strings, and language names; Spanish uses formal usted register throughout
• app/[locale]/layout.tsx wraps the locale segment with NextIntlClientProvider; app/[locale]/page.tsx renders the full landing page using useTranslations so /es/ serves every hero string, nav label, stat, and program card in Spanish
• components/ui/language-selector.tsx is a dropdown showing flag and language name; selecting a locale navigates to /es or / and persists the preference to localStorage
• LanguageSelector is added to both the desktop and mobile sections of components/landing/Navbar.tsx
• lib/glossary.ts defines bilingual (en/es) definitions for escrow, XLM, Soroban, Freighter, gas fee, smart contract, and testnet
• components/ui/glossary-term.tsx wraps any inline text and shows a tooltip on hover or focus with the definition in the current locale, auto-detected from the URL pathname

Test plan

• Run npm install to pull in next-intl
• GET / loads the landing page in English
• GET /es loads the landing page with all hero and nav strings in Spanish
• Setting Accept-Language: es header on GET / redirects or serves Spanish
• Language selector in the navbar toggles between / and /es and persists to localStorage
• Hovering or focusing the word escrow on a page that uses GlossaryTerm shows the tooltip in the correct locale
• Existing routes /connect, /escrow, /wallet, /history are unaffected

closes #753
closes #754
closes #757
closes #758

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