refactor: drop Qwen3 CoreML ASR, add local-engine cloud polish toggle

Rolls back the v0.2.0 Qwen3 CoreML on-device ASR stack and replaces the
'local engine' UX with iOS 26 SpeechAnalyzer + DictationTranscriber only.

The 'Cloud polish after ASR' toggle (ProviderConfig.localModeCloudPolishEnabled)
lets users opt into a post-ASR DeepSeek round-trip from the local engine.
Defaults to off so the local engine stays genuinely local. New PolishError.missingAPIError
surfaces an inline 'fill in your key' warning when the toggle is on but the
Keychain is empty. DeepSeek preset default model bumped to deepseek-v4-flash.

Deleted:
  - OSGKeyboard/ThirdParty/Qwen3Speech/ (74 files, ~16k LoC)
  - OSGKeyboard/Services/ModelManager.swift (492)
  - OSGKeyboard/Services/OnDeviceModelWarmup.swift (197)
  - OSGKeyboard/Services/Qwen3ASRService.swift (257)
  - OSGKeyboard/Services/ModelDownloadSourcePicker.swift (126)
  - OSGKeyboard/Views/OnDeviceModelsView.swift (184)
  - OSGKeyboard/Views/DownloadConfirmSheet.swift (96)
  - OSGKeyboardShared/Models/OnDeviceModel.swift (140)
  - OSGKeyboardShared/Services/OnDeviceModelStatus.swift (104)
  - Qwen3ASRServiceProvider registration in OSGKeyboardApp
  - Qwen3Speech package declaration in project.yml
  - 5 .qwen3ASR enum / branch reference sites in HomeView, OnboardingView,
    LocalEngineSettingsRows, FlowSessionManager, ASRService, EngineServiceLabel
  - Two pre-existing Swift 6 strict-concurrency errors in
    LiveDictationController + FlowSessionManager (the weak [weak self] in
    detached-task MainActor.run blocks) that were blocking clean builds

Added:
  - LocalModelsGroup: 'Built-in iOS SpeechAnalyzer' badge + 'Cloud polish
    after ASR' Switch toggle
  - PolishingService: honour localModeCloudPolishEnabled; new .missingAPIKey
    error case with localised warning
  - AppGroupStore.localModeCloudPolishEnabled (mirrored into App Group
    so the keyboard extension honours the toggle during live dictation)
  - SettingsView: show provider/api sections when local-mode cloud polish
    is on so the user can paste a DeepSeek key
  - FlowSessionManager: route through PolishingService for local + polish-on
    flow; translate missingAPIKey into a polished warning
  - KeyboardViewController: handle PolishingService.PolishError.missingAPIKey
    in the keyboard-side live polish path
  - CHANGELOG v0.2.1: documents the rollback + new toggle
  - README.md / README.zh.md: engine matrix section, data flow note

Verified: xcodebuild -scheme OSGKeyboard -destination 'generic/platform=iOS Simulator'
build succeeds under SWIFT_STRICT_CONCURRENCY=complete.

OSGKeyboardShared/Services/PolishingService.swift

@@ -5,9 +5,17 @@
 // to produce polished, well-punctuated text. Falls back to the raw transcript
 // if the LLM call fails or times out.
 //
-// Cloud engine always runs the LLM polish step (settings no longer expose
-// off / transcribe). Local engine (`engineMode == "local"`) is ASR-only —
-// the raw transcript is returned unchanged and cloud API settings are ignored.
+// Engine matrix:
+//   - `engineMode == "cloud"`  → always polish (cloud engine's whole point).
+//   - `engineMode == "local"`,
+//     `localModeCloudPolishEnabled == false` → ASR-only, return raw.
+//   - `engineMode == "local"`,
+//     `localModeCloudPolishEnabled == true`  → polish via the user's LLM
+//     (DeepSeek by default). The local engine gains stronger accuracy on
+//     noisy / dialectal Chinese at the cost of one cloud round-trip.
+//     If the user hasn't entered an API key the call falls back to the
+//     raw transcript and surfaces a warning so the keyboard can show
+//     the "fill in your key" hint.
 
 import Foundation
 
@@ -16,6 +24,11 @@ public actor PolishingService {
     public enum PolishError: Error, Equatable {
         case noTranscript
         case timeout
+        /// v0.2.0: local engine + cloud-polish-on, but the user hasn't
+        /// saved an API key in the Keychain. Caller surfaces an Alert
+        /// telling them to fill it in; we deliver the raw transcript
+        /// so no data is lost.
+        case missingAPIKey
     }
 
     private let store: AppGroupStore
@@ -43,9 +56,17 @@ public actor PolishingService {
         let trimmed = raw.trimmingCharacters(in: .whitespacesAndNewlines)
         guard !trimmed.isEmpty else { throw PolishError.noTranscript }
 
-        // Local engine: ASR-only — no on-device or cloud polish.
+        // Local engine: ASR-only unless the user opted into cloud
+        // polish via `localModeCloudPolishEnabled`. The cloud polish
+        // path still requires an API key; if the Keychain is empty we
+        // fall back to the raw transcript and throw `missingAPIKey`
+        // so the UI can surface the "fill in your key" hint.
         if store.engineMode == "local" {
-            return trimmed
+            guard store.localModeCloudPolishEnabled else { return trimmed }
+            guard !store.apiKey.isEmpty else {
+                throw PolishError.missingAPIKey
+            }
+            return try await polishRemote(trimmed)
         }
 
         return try await polishRemote(trimmed)