[Loc][MWPW-188683] Support config-stage sheet and defaultlocales column for locale selection

[maagrawal16]

These are the changes made to support Lingo in the Milo Studio Localization workflow:

config-stage sheet support

• fetchLocales now accepts an optional configName parameter (defaults to 'config')
• When the env is stage or local, the tool first attempts to fetch from config-stage.json, which allows tenants to maintain stage-specific locale configurations
• Falls back gracefully to config.json if config-stage is not present for a given tenant, so existing tenants are unaffected
• env is now resolved before fetchLocaleDetails() is called in locui-create.js to ensure the correct config sheet is fetched on initialization

defaultlocales column support

• Locale sheets can now include an optional defaultlocales column per language, allowing tenants to define a subset of livecopies that should be pre-selected when a language is chosen
• When defaultlocales is present, only those locales are activated on language select (instead of all livecopies)
• Deselecting a language still removes all livecopies as before
• setSelectedLocalesAndRegions and getLanguageDetails in utils.js also respect defaultlocales when computing active/selected locales

defaultlocales in translation workflow

• For translation projects, only the defaultlocales for each language are passed in the locales payload (instead of an empty list)
• No frontend behavior change — this is a data-only update to ensure the correct locales are sent downstream

defaultlocales in region selection

• When a region is selected (e.g. Americas), all locales from the region still appear in the Selected Locales panel, but only locales that match a language's defaultlocales are activated
• For example, selecting Americas adds ca (English) and ca_fr (French) to the panel, but since French has defaultlocales = fr, only ca is active — ca_fr appears but is not pre-selected
• Deselecting a region still removes all its locales as before

Resolves: MWPW-188683

Test URLs:

• Before: https://main--milo--adobecom.aem.page/?martech=off
• After: https://mwpw-188683--milo--maagrawal16.aem.page/?martech=off