feat(frontend): add Vue I18n 9, locale files for home page

[ChinmayShringi]

feat(frontend): Add Vue I18n 9 and internationalize the Home page

This PR is part 1 of the page internationalization series:

• Introduce vue-i18n@9; add zh-CN.json and en.json under src/locales (containing home., common. namespaces)
• Create src/i18n.js: use createI18n, read locale from localStorage, fallback to en, and implement applyLocaleToDocument()
• In main.js: app.use(i18n), apply current language to document on initialization
• Home.vue: replace UI text with $t('home.xxx'); add language toggle (Chinese / English) to navbar; on switch, update localStorage, document.documentElement.lang, and <title>
• Add docs/I18N-BRANCHES.md explaining branch planning for subsequent Process, History, Steps, Report, Interaction modules

See /MiroFish/docs/I18N-BRANCHES.md for more details.
A language option has been added to the top-right corner of the navbar.

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Original PR: 666ghj/MiroFish#215
Original Author: @isabelccc