Fix incorrect GTM container IDs in xplat JP builds

[Copilot]

Xplat JP builds (React, WebComponents, Blazor) were resolving EN GTM container IDs instead of JP-specific ones because the fallback in getGtmContainerId() had no locale awareness — it was a flat map keyed only by build mode.

Changes

• src/lib/platform-context.ts — Restructured GTM_CONTAINER_ID_DEFAULTS from Record<string, string> to Record<string, Record<string, string>> (locale → mode → container ID), and updated getGtmContainerId() to select the correct locale bucket using LANG_CODE before resolving by build mode. Unknown locales fall back to en defaults.

// Before
const GTM_CONTAINER_ID_DEFAULTS: Record<string, string> = {
    production: 'GTM-T65CF7',
    staging: 'GTM-NCKNPN',
    development: 'GTM-NCKNPN',
};

// After
const GTM_CONTAINER_ID_DEFAULTS: Record<string, Record<string, string>> = {
    en: { production: 'GTM-T65CF7', staging: 'GTM-NCKNPN', development: 'GTM-NCKNPN' },
    jp: { production: 'GTM-KVNSWJ', staging: 'GTM-WLWSDK', development: 'GTM-NNHVMC7' },
};

Resolution priority is unchanged: GTM_CONTAINER_ID env var → GTMContainerId from environment.json → locale-aware built-in defaults.

• Fixes Incorrect GTM container ID in xplat JP builds #500