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.

OSGKeyboard/Views/OnboardingView.swift

@@ -742,6 +742,11 @@ private struct APISetupPage: View {
                     // step through DeepSeek, so the constraint is gone).
                     // Wrapped in the same surface card chrome as the
                     // APISettingsCard above for visual symmetry.
+                    //
+                    // v0.2.1 final review (topbar cleanup pass): the
+                    // surface card chrome is owned by `translationSection`
+                    // itself, so both branches get it for free — no
+                    // per-branch duplication.
                     if config.isTranslationRowVisible {
                         translationSection
                             .padding(.horizontal, Spacing.lg)

OSGKeyboard/Views/ProviderPickerSection.swift

@@ -10,15 +10,20 @@ struct ProviderPickerSection: View {
     @ObservedObject var config: ProviderConfig
 
     var body: some View {
+        // v0.2.1 follow-up: filter out presets marked as
+        // `isUserSelectable == false` so a future "DeepSeek key
+        // pre-fill" preset (or similar) can ship in `presets` without
+        // showing up in the picker.
+        let visiblePresets = LLMProvider.presets.filter { $0.isUserSelectable }
         VStack(spacing: 0) {
-            ForEach(Array(LLMProvider.presets.enumerated()), id: \.element.id) { index, provider in
+            ForEach(Array(visiblePresets.enumerated()), id: \.element.id) { index, provider in
                 Button {
                     select(provider)
                 } label: {
                     row(provider, selected: provider.id == config.providerId)
                 }
                 .buttonStyle(.plain)
-                if index < LLMProvider.presets.count - 1 {
+                if index < visiblePresets.count - 1 {
                     Divider().background(palette.divider)
                 }
             }