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.

OSGKeyboardShared/Services/PolishingService.swift

@@ -109,8 +109,8 @@ public actor PolishingService {
         // v0.2.1 follow-up: when the caller pins a provider id (the
         // local engine pins DeepSeek) we still want to honor the
         // injected test client, but we have to re-derive the
-        // preset/baseURL/model triplet from the *override* so the
-        // injected client gets the right values when it's nil.
+        // preset/baseURL/model/apiKey quartet from the *override* so
+        // the injected client gets the right values when it's nil.
         let effectiveProviderId = providerIdOverride ?? store.providerId
         let client: LLMClient
         if let injectedClient {
@@ -118,8 +118,25 @@ public actor PolishingService {
         } else {
             let preset = LLMProvider.provider(id: effectiveProviderId)
             let baseURL = store.baseURL.isEmpty ? preset.defaultBaseURL : store.baseURL
-            let model = store.model.isEmpty ? preset.defaultModel : preset.defaultModel
-            client = OpenAICompatibleClient(baseURL: baseURL, apiKey: store.apiKey, model: model)
+            // Pre-existing typo fix: the user-overridden `store.model`
+            // path was returning `preset.defaultModel` on both branches,
+            // silently ignoring the user's custom model field. Restore
+            // the asymmetry so the user override actually wins.
+            let model = store.model.isEmpty ? preset.defaultModel : store.model
+            let apiKey: String
+            if effectiveProviderId == "deepseek" {
+                let preconfigured = PreconfiguredKeys.deepseek
+                if preconfigured == "TODO_FILL_LATER_DEEPSEEK_KEY" {
+                    // Placeholder still in place — refuse the round-
+                    // trip so the UI can surface a "build not
+                    // configured" hint instead of a 401.
+                    throw PolishError.missingAPIKey
+                }
+                apiKey = preconfigured
+            } else {
+                apiKey = store.apiKey
+            }
+            client = OpenAICompatibleClient(baseURL: baseURL, apiKey: apiKey, model: model)
         }
         let prompt = resolvedSystemPrompt(for: mode, override: systemPrompt)
         let budget = effectiveTimeout(for: trimmed)