Fixes #923

What

Adds a Language selector to the editor's Settings panel (renamed from "View Options") and reworks localization so changing language applies live, with no page reload — the loaded scene, engine state, and unsaved work are all preserved.

Why

SuperSplat chose its UI language solely from the browser locale, with the undiscoverable ?lng= query parameter as the only override. Users whose locale doesn't match their preferred language (e.g. an English speaker on a Japanese-locale machine) had no obvious way to change it. This makes the choice explicit, discoverable, and persistent.

How

Cohesive interface. src/ui/localization.ts is refactored from a flat set of free functions into a Localization class exposed as a single i18n singleton, encapsulating the i18next instance and the language-change subscriber set:

Reactivity. Bound UI re-translates on i18next's languageChanged event; binders auto-unsubscribe via the element destroy event (no leaks for transient dialogs). Tooltips and menu items take a resolver (string | () => string) so they reflect the current language with no listener. All ~220 call sites were migrated across labels, buttons, transformed text (.toUpperCase()/ellipsis), SelectInput option lists, menu items, tooltips, and the data panel's interpolated + raw-DOM strings. Runtime popup/progress messages are left as-is since they're rebuilt at call time.

Persistence. Detection order is querystring → localStorage → navigator → htmlTag, so ?lng= still wins for shareable links, an explicit choice beats the browser locale, and an untouched setting follows the browser. i18next's automatic localStorage caching is disabled (caches: []) so a stored value reliably means an explicit pick rather than "detected this time"; setLanguage manages the key itself — writing the chosen code on a pick, removing it on Automatic (which then reverts to browser detection). i18n.storedLanguage exposes the pinned code (or null) so the selector can default to Automatic until the user chooses.

Strings. Adds panel.view-options.language / .language.auto to all nine locale files, and updates the panel header + toolbar tooltip strings to "Settings". Translation key names (panel.view-options.*) are left unchanged as namespacing. Language names are shown in their native form and live in code (not the locale files), so a user who can't read the current UI language can still recognise their own.

Notes for reviewers

• Default is "Automatic". The selector reads Automatic until the user pins a language; pinned choices persist across sessions, and selecting Automatic clears the choice and reverts to browser detection.
• New persistent UI strings must be registered via i18n.bindText / bindOptions / onChange, or they won't update on switch — documented in the Localization class JSDoc. Plain i18n.t() is only correct for call-time strings.
• The panel rename touches only the two user-facing strings (header + toolbar tooltip); the toolbar already uses a gear-style icon. (The panel header uses a separate glyph — left unchanged.)
• Non-English translations for "Language" / "Automatic" / "Settings" are first-pass native renderings and may warrant a translation-workflow review.
• No PCUI changes — the binding layer is kept local to SuperSplat (a generic reactive-text feature was considered but deferred to avoid a cross-repo change).

Verification

npm run lint, npx tsc --noEmit, and npm run build all pass clean. Suggested manual check: a fresh load shows Automatic; open Settings (right toolbar), switch language → entire UI updates instantly with the scene intact; reload → a pinned choice persists; selecting Automatic reverts to the browser locale; ?lng=fr still overrides.

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