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/Views/SettingsView.swift

@@ -74,6 +74,9 @@ struct SettingsView: View {
                                 apiSection
                             }
                             languageAndModelsSection
+                            if config.engineMode == "local" {
+                                localEngineSettingsSection
+                            }
                             if config.engineMode == "cloud" {
                                 systemPromptLinkSection
                             }
@@ -131,8 +134,9 @@ struct SettingsView: View {
                 // now sits above the cloud-polish toggle / local models
                 // block so the row that maps to microphone input comes
                 // first, the row that maps to post-processing comes
-                // second, and translation (post-post-processing) sits at
-                // the bottom.
+                // second. Translation moved into the local-engine
+                // group (see `localEngineSettingsSection`) so the local
+                // engine reads as one cohesive card on its own.
                 LocalePickerRow(
                     locales: effectiveLocales,
                     selection: Binding(
@@ -140,14 +144,6 @@ struct SettingsView: View {
                         set: { config.localeId = $0 }
                     )
                 )
-                if config.engineMode == "local" {
-                    Divider().background(palette.divider)
-                    LocalModelsGroup(config: config)
-                }
-                Divider().background(palette.divider)
-                if config.isTranslationRowVisible {
-                    TranslationPickerRow(config: config, isVisible: true)
-                }
             }
             .background(palette.surface, in: RoundedRectangle(cornerRadius: Radius.large, style: .continuous))
             .overlay(
@@ -178,6 +174,19 @@ struct SettingsView: View {
         }
     }
 
+    /// v0.2.1 follow-up: dedicated section for the local engine's
+    /// settings (cloud-polish toggle + translation row). Renders only
+    /// when `engineMode == "local"` so the cloud-engine user doesn't
+    /// see rows that are inert for them. The translation row lives
+    /// inside `LocalModelsGroup` so it shares the group's surface card
+    /// chrome — see `LocalEngineSettingsRows.swift` for the layout.
+    private var localEngineSettingsSection: some View {
+        VStack(alignment: .leading, spacing: SettingsListMetrics.sectionLabelSpacing) {
+            sectionHeader("settings.localEngine.title")
+            LocalModelsGroup(config: config)
+        }
+    }
+
     private var providerSection: some View {
         VStack(alignment: .leading, spacing: SettingsListMetrics.sectionLabelSpacing) {
             sectionHeader("settings.provider.title")