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.

OSGKeyboardShared/Models/LLMProvider.swift

@@ -14,6 +14,12 @@ public struct LLMProvider: Identifiable, Codable, Hashable, Sendable {
     public let apiKeyURL: URL?
     /// Optional short blurb shown under the provider name in the picker.
     public let blurb: String?
+    /// Whether this preset should appear in user-facing provider pickers
+    /// (settings / onboarding). Defaults to `true` so the existing
+    /// `presets` array keeps its public surface area; future passes can
+    /// mark e.g. a DeepSeek key-pre-fill preset as `false` to hide it
+    /// from the picker without touching call sites.
+    public let isUserSelectable: Bool
 
     public init(
         id: String,
@@ -21,7 +27,8 @@ public struct LLMProvider: Identifiable, Codable, Hashable, Sendable {
         defaultBaseURL: String,
         defaultModel: String,
         apiKeyURL: URL? = nil,
-        blurb: String? = nil
+        blurb: String? = nil,
+        isUserSelectable: Bool = true
     ) {
         self.id = id
         self.name = name
@@ -29,6 +36,7 @@ public struct LLMProvider: Identifiable, Codable, Hashable, Sendable {
         self.defaultModel = defaultModel
         self.apiKeyURL = apiKeyURL
         self.blurb = blurb
+        self.isUserSelectable = isUserSelectable
     }
 
     public static let presets: [LLMProvider] = [