feat(translation-polish-2): local-engine dedicated section + PreconfiguredKeys for DeepSeek

- Add PreconfiguredKeys.swift: placeholder constant for the DeepSeek
  API key the local engine uses. DEBUG build asserts at launch when
  the placeholder is still in place so nobody ships an always-401
  build by accident. Replace TODO_FILL_LATER_DEEPSEEK_KEY before
  distributing.

- PolishingService.polishRemote:
  * Adopted (A) path: when effective provider is DeepSeek, take the
    apiKey from PreconfiguredKeys.deepseek instead of store.apiKey.
    Refuse the round-trip with PolishError.missingAPIKey when the
    placeholder is still in place.
  * Adopted fix for pre-existing typo: store.model.isEmpty ?
    preset.defaultModel : store.model (the right-hand side was
    defaultModel on both branches, silently ignoring the user's
    custom model field).

- LocalEngineSettingsRows.LocalModelsGroup:
  * Dropped the inline 'uses DeepSeek' caption — it added visual
    weight without telling the user anything they couldn't infer.
  * Translation row now lives inside the group so the local engine
    reads as one cohesive card.
  * Group owns its surface card chrome (palette.surface + rounded
    border) — callers no longer need to wrap it externally.

- SettingsView:
  * languageAndModelsSection: dropped the trailing TranslationPickerRow
    and the inner LocalModelsGroup (now lifted to its own section).
  * New localEngineSettingsSection: renders only when
    engineMode == 'local', wrapping LocalModelsGroup with a
    'settings.localEngine.title' header.
  * body: added the new section between languageAndModelsSection and
    the cloud-only systemPromptLinkSection.

- Localizable.strings (en + zh-Hans):
  * Removed settings.localModels.cloudPolish.caption (no references).
  * Added settings.localEngine.title = 'Local engine' / '本地引擎'.

DeepSeek key pre-fill tripwire + local-engine UI cohesion. No main
merge, no PR.

OSGKeyboard/en.lproj/Localizable.strings

@@ -119,7 +119,7 @@
 "settings.localModels.readiness %lld %lld" = "%lld/%lld ready";
 "settings.localModels.cloudPolish.title" = "Cloud polish after ASR";
 "settings.localModels.cloudPolish.subtitle" = "Sends the transcript to your configured cloud LLM (DeepSeek by default) for cleanup. Enable only when iOS speech recognition struggles — noisy far-field audio, strong accents, etc. Requires a DeepSeek API key.";
-"settings.localModels.cloudPolish.caption" = "Uses DeepSeek to polish transcripts in local mode.";
+"settings.localEngine.title" = "Local engine";
 "settings.language.subtitle.cloud" = "Recognition language and text processing mode.";
 "settings.language.subtitle.local" = "Recognition language.";
 "settings.mode.title" = "Mode";