feat: on-device translation of item text (P13b-2)

[blokzdev]

What & why

Second half of P13b — on-device translation. A "Translate…" action on item detail translates an item's description + transcript into the user's language, fully on-device via ML Kit. No Google Play Services — language models download over HTTPS (~30 MB each, English pivot) and run offline after — fitting GrabBit's sideloaded/de-Googled posture. Translation is ephemeral (no cache, no schema change).

Changes

• Deps: google_mlkit_translation + google_mlkit_language_id (both MIT), recorded in docs/SPEC.md.
• Engine seam (mirrors P13b-1 OcrEngine): TranslationEngine + MlKitTranslationEngine (Android — OnDeviceTranslator + OnDeviceTranslatorModelManager + LanguageIdentifier) + UnavailableTranslationEngine + factory/provider; graceful off-Android. Pure translateLanguageForCode (BCP↔TranslateLanguage); new translateFailed error code.
• Flow: overflow Translate… → target-language picker (default = app language, reusing the caption-language picker) → detect source → action-driven ~30 MB pack-download confirm (Wi-Fi) → translate. A pure translateReadiness decision (unavailable/notDetected/alreadyInTarget/needsDownload/ready) drives it.
• State: a screen-scoped autoDispose itemTranslation(itemId) controller; description + transcript render translated with a "Translated from · Show original" toggle. Added a one-shot MetadataRepository.metadataForItem.
• Scope: description + transcript only — the derived AI/TextRank summaries stay in the source language (→ BACKLOG).

Tests

dart format clean · flutter analyze No issues · flutter test 791 passed — including engine availability + BCP mapping, the translateReadiness truth table, and the controller with a fake TranslationEngine (translate populates description/transcript + langs; Show original toggle; description-only item). No schema change.

Honest notes

• Owed APK spot-check (VERIFICATION → P13b-2): the native ML Kit translate/language-id calls + the ~30 MB pack download can't be CI-tested; the overflow→picker→download→translated-render widget flow is APK-verified (consistent with the OCR/whisper/generation boundary).
• Ephemeral by design — re-translating re-runs ML Kit (instant once the pack is present). Caching, translating the summaries, and a language-pack management/delete UI are logged in BACKLOG.
• A design correction made mid-build: the controller first read the metadata via the StreamProvider.future, which timed out in ProviderContainer tests (the temporary ref.read subscription was disposed before the drift stream emitted). Switched to a one-shot metadataForItem read — correct for a command and reliably testable.

This completes P13b's translation; P13b-3 (opt-in auto-OCR on download) remains a planned follow-up.

https://claude.ai/code/session_013JoYmLCosYt5tQ8qwdbL1T

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