feat(translation-polish): dual-engine translation UX + provider picker filter + topbar cleanup

- Add LLMProvider.isUserSelectable (default true) and filter
  ProviderPickerSection on it; next pass can hide non-user presets
  (e.g. a future DeepSeek key-preset) without changing call sites.
- KeyboardRootView: hide TranslationChip when off (matches user's
  mental model of an opt-in feature), drop the 'warming' branch
  (Qwen3 download UX was removed with the backend in v0.2.0), unify
  chip pill height to minHeight 28 + vertical 6 for visual rhythm
  across all topbar chips.
- TranslationChip: drop isLocal warning path — both engines now
  run the translate-and-polish step (local routes through DeepSeek
  via ProviderConfig.localModeProviderId).
- OnboardingView: cloud engine branch now wraps the translation
  row in the same surface card chrome as the local branch.
- Strings: drop keyboard.models.warming (no longer referenced).

DeepSeek key pre-fill deferred to a follow-up.

OSGKeyboardExt/en.lproj/Keyboard.strings

@@ -121,7 +121,6 @@
 "keyboard.placeholder.cloudBadge" = "Cloud";
 "keyboard.models.notDownloaded" = "On-device models not downloaded";
 "keyboard.models.downloadHint" = "Open OSGKeyboard to download models";
-"keyboard.models.warming" = "Loading models…";
 "keyboard.rec" = "REC";
 "keyboard.space" = "Space";
 "keyboard.denied.mic" = "Mic denied";