Summary

Vite reported a 1.4 MB index-*.js chunk and three INEFFECTIVE_DYNAMIC_IMPORT warnings. Non-English locales were eagerly embedded, and YouTube playback dynamically imported Tauri APIs that the startup path already loads statically, so those import() calls never became real chunks.

This change lazy-loads non-English locale JSON through i18next.changeLanguage, and isolates the YouTube watch native surface in a module that is imported only when YouTube actually plays. PlaybackStage uses the same static Tauri emit / getCurrentWindow path as the rest of startup.

Changes

• Bundle English only at startup; other locale JSON files load on demand.
• Move createDefaultYoutubeWatchNativeSurface to src/playback/youtube-watch-native.ts and load it from the YouTube transport.
• Keep YouTube host bounds reporting on the existing static Tauri window/event APIs.

Acceptance criteria

• Vite production build emits no INEFFECTIVE_DYNAMIC_IMPORT warnings.
• The main index-*.js chunk stays under the 1000 kB warning limit.
• Switching to a non-English app language still loads that locale’s strings and updates document.documentElement.lang.
• YouTube watch host creation still goes through the native surface; jsdom tests keep working without a live Tauri runtime.

Test plan

• node --run lint and node --run build — local pre-push lint passed; production build emitted index at 871.23 kB gzip 248.74 kB, a youtube-watch-native chunk, per-locale chunks, and no Vite warnings
• pnpm vitest run — 220 files / 2417 tests passed; patch coverage 82.6%
• cd src-tauri && cargo clippy --all-targets -- -D warnings — skipped, no Rust changes
• cd src-tauri && cargo nextest run — skipped, no Rust changes
• node --run check:i18n — skipped, no UI copy added; locale JSON files are unchanged
• docs/references/contracts/*.md updated — not required; no public IPC command, payload, or event change
• Product-standard profiles: Language, terminology, and data (BCP-47 tags and document.lang unchanged; on-demand locale load covered in src/lib/i18n.test.ts) and Lifecycle, quality, and testing (chunk-size / ineffective-dynamic-import acceptance above). PlaybackStage still reports YouTube host bounds; no keyboard or widget contract change.

Residual risk

First switch to a non-English language now waits on a locale JSON chunk. Startup still hydrates settings through the existing await changeLanguage(...) path, so the English fallback window is the same as before plus that chunk fetch.

Related issues

Stacked on #419 (feat/418-online-sources). No separate issue.