feat(frontend): Load geonetwork-ui translations at runtime

[tkohr]

The frontend now uses its own TranslateLoader, which merges the geonetwork-ui translations (copied to assets/i18n/gn-ui/ at build time) with datafeeder's. Datafeeder values take precedence, and empty values are ignored.

This removes the duplicated gn-ui markers (metadata.i18n.ts) and their translations from translations/*.json. Languages other than en/fr now get gn-ui's translations instead of falling back to English.

gn-ui has been updated to align all translation values that corresponded to markers kept in the deleted metadata.i18n.ts.

Translation overrides

gn-ui translation values can still be overridden as before by adding markers to components that use gn-ui components. These overrides will be applied to datafeeder in general.

The overrides currently present in the datafeeder (and that were not touched in this PR) are:

Key	Lang	Datafeeder	gn-ui
input.file.selectFileLabel	en	Choose a file	Select a file ({sizeMB} MB max.)
input.file.orInputUrl	en	Or import a file from a link	or create a link to an external resource
input.file.selectFileLabel	fr	Choisissez un fichier	Sélectionnez un fichier ({sizeMB} Mo max.)
input.file.dropFileLabel	fr	ou déplacez le ici	ou glissez-le ici
input.file.orInputUrl	fr	Ou importez un fichier à partir d'un lien	ou créez un lien vers une ressource externe

If these were not done intentionally they can still (partially) be dropped.

Note: A follow-up PR (that will load the gn-ui default.toml config) will allow to override translations by datafeeder instance.