Add Chinese and English Web localization

[sam2tom]

Summary

• add i18next/react-i18next with English and Simplified Chinese feature namespaces
• follow browser language preference, persist explicit selection, and migrate the legacy locale preference
• translate the Web shell and complete Agents, Sessions, Templates, Vaults, System, Sandbox, connection, Dashboard, Runtime, and Source Files workflows
• format dates and numbers with the selected application locale and verify locale key parity

Validation

• pnpm --filter @agents-core-web/web typecheck
• pnpm --filter @agents-core-web/web test (71 files, 692 tests)
• pnpm --filter @agents-core-web/web build
• git diff --check
• manual English/Chinese switching and persistence in the browser

Full repository gate

make check could not start because PARSAR_AGENTS_API_TEST_DATABASE_URL is not set in the login shell. No secret value was printed.

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