fix(i18n): shorten nav labels and complete translations in all locales
Nav bar labels that overflow their tab slot on phones are shortened to idiomatic short forms: fr (Bibliothèque, Téléchargement, Recherche), ru/bg/it (Live TV), pl (Home). All 21 locales get the ~280 keys that were empty (falling back to English at runtime): explore detail/badges/stats, fileInfo, startup repair flow, mediaMenu delete dialogs, addServer, rating sources, downloads sync-rule removal, and more. settings.displayScale was missing entirely in 16 locales; the new exoplayer playbackBuffer keys (upstream feat) are translated too. Fixes mis-translations found in review: es/zh/zh-Hant sidecar-format wording, sv adaptation, nb transcoding. Regenerated with dart run slang; translation hygiene and i18n tests pass. Close #1823
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