[SuperAppAdminPortal] Add multi-language support (English, Sinhala, Tamil)

[MohamadNazik]

Description:

The admin portal (superapp-admin-portal) currently has all UI strings hardcoded in English. There is no internationalization (i18n) framework or language switching mechanism available to administrators. Sri Lanka's three official/national languages — English, Sinhala (සිංහල), and Tamil (தமிழ்) — should all be supported, allowing admin users to switch the portal language.

Current Behaviour

• All user-facing strings (page titles, table headers, form labels, button text, dialogs, error/success messages, etc.) are hardcoded in English directly inside React components.
• No i18n library is installed.
• No language selector exists in the portal UI.

Expected Behaviour

• Admins should be able to select their preferred language from the portal header or a settings area.
• The entire portal UI should re-render in the selected language.
• The selected language preference should be persisted in localStorage across sessions.
• The portal should default to the browser's locale if it matches a supported language; otherwise, default to English.

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Implementation Guidance

1. Install i18n Dependencies

Install an i18n library compatible with React:

• i18next + react-i18next
• i18next-browser-languagedetector (for browser locale detection)

2. Create Translation Files

Add a superapp-admin-portal/src/locales/ directory with JSON translation files:

superapp-admin-portal/src/locales/
├── en.json    # English (default)
├── si.json    # Sinhala (සිංහල)
└── ta.json    # Tamil (தமிழ்)

3. Initialize i18n

Create superapp-admin-portal/src/lib/i18n.ts to configure i18next with:

• Fallback language: en
• Browser language detection via i18next-browser-languagedetector
• Persistence to localStorage

4. Add a Language Selector to the Layout

Add a language switcher dropdown (MUI Select or Menu) to the portal's header/app bar (src/components/layout/) with options:

• 🇬🇧 English
• 🇱🇰 සිංහල (Sinhala)
• 🇱🇰 தமிழ் (Tamil)

5. Extract All Hardcoded Strings

Replace all hardcoded English strings with translation keys (t('key')) across:

Pages:

• src/pages/MicroApps.tsx — Micro Apps management
• src/pages/Users.tsx — User management
• src/pages/Login.tsx — Login page
• src/pages/AccessDenied.tsx — Access denied page
• src/pages/ComingSoon.tsx — Coming soon page
• src/pages/NotFound.tsx — 404 page

Components:

• src/components/layout/ — Navigation sidebar, app bar, header
• src/components/common/ — Shared UI components
• src/components/ui/ — UI primitives

Other:

• src/App.tsx — Root app component (routing labels, etc.)
• Any dialog/modal text, toast messages, validation messages

6. MUI Locale Integration

Configure MUI's ThemeProvider with the appropriate locale for built-in component text (e.g., table pagination "Rows per page", date picker labels).

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Related Issues:

• [Frontend] Add multi-language support (English, Sinhala, Tamil) #89