feat(keyboard): in-keyboard onboarding overlay + AppContext chip

Two long-standing UX papercuts, fixed without leaving the keyboard:

1. **First-launch onboarding inside the keyboard** — iOS keyboard
   extensions *cannot* programmatically switch back to the previous
   app after jumping out, so the old flow (jump to host app → user
   has to manually navigate back) was 5+ taps of friction. The new
   `KeyboardOnboardingOverlay` keeps the user inside the keyboard
   for steps 1 (welcome), 2 (mic permission), 3 (speech permission),
   and 5 (API key hint). The only step that *must* leave is step 4
   ("Enable Keyboard"), which jumps to `Settings.app` via
   `UIApplication.openSettingsURLString` — on return,
   `viewWillAppear` calls `autoAdvancePastKeyboardSetupStepIfNeeded`
   which silently advances past that step if the keyboard is now
   enabled. Net UX: user types in their app, keyboard walks them
   through setup, normal UI appears as soon as setup is done.

2. **Per-app context chip on the keyboard top bar** — the v0.3.0
   intelligent-prompt pipeline already adapted tone to detected
   context (code/email/chat/document), but without a UI cue the user
   had no way to know which mode was active or override the heuristic
   when it guessed wrong. The new `AppContextChip` surfaces the
   detected context; tap-to-override writes back to
   `AppGroupStore.setDetectedAppContext` so the next LLM call
   picks up the new tone immediately. Wired into `pressBegan` so
   the chip updates in real time as the user types into different
   fields.

### Files added
- `OSGKeyboardExt/Views/AppContextChip.swift` — chip + Menu override
- `OSGKeyboardExt/Views/KeyboardOnboardingOverlay.swift` — 5-step overlay
- `OSGKeyboardTests/KeyboardOnboardingOverlayTests.swift` — round-trip + enum surface tests

### Files modified
- `OSGKeyboardShared/Services/KeyboardState.swift`
  + `hasCompletedOnboarding`, `onboardingPage`, `appContext`
  + `setAppContext`, `advanceOnboarding`, `completeOnboarding`
  + `requestMicPermission`, `requestSpeechPermission`, `openSystemSettings`
- `OSGKeyboardShared/Services/AppGroupStore.swift`
  + `hasCompletedOnboarding` / `onboardingPage` accessors (mirror of
    `ProviderConfig` keys, so the keyboard extension never has to
    instantiate the host-app config)
- `OSGKeyboardExt/KeyboardViewController.swift`
  + action hooks wired (`installStateActions`)
  + `syncOnboardingStateFromAppGroup` / `syncAppContextFromAppGroup`
    called on `viewWillAppear` and `loadPersistedConfig`
  + `autoAdvancePastKeyboardSetupStepIfNeeded` for the silent
    "jump out → come back" flow
  + `openSystemSettingsFromExtension` opens `Settings.app` via
    `UIApplication.openSettingsURLString` (the only system URL
    the extension is allowed to open)
  + `detectAndStoreAppContext` mirrors to `state.appContext` so
    the chip updates without waiting for `viewWillAppear`
- `OSGKeyboardExt/Views/KeyboardRootView.swift`
  + overlay mounted in `ZStack` over normal UI (animated)
  + AppContextChip in top bar (hidden during onboarding)
- `OSGKeyboardExt/{en,zh-Hans}.lproj/Keyboard.strings`
  + onboarding copy + chip labels (35 keys per language)

### iOS sandbox notes (kept here for posterity)
- Keyboard extensions **cannot** present AVAudioSession /
  SFSpeechRecognizer permission dialogs directly. The overlay's
  step 2/3 buttons optimistically advance; the actual permission
  is granted when the user first opens the host app (which the
  step-5 "Open OSGKeyboard" button triggers). This is the same
  pattern the previous "jump to host app" flow used — just
  without the broken return trip.
- `UIApplication.openSettingsURLString` is the only system URL
  reachable from `extensionContext.open`. Both step 4 and the
  "Open Settings" button route through `HostAppLauncher` so the
  responder-chain fallback also kicks in if needed.

Co-authored-by: Mavis <Mavis@hkgood.dev>

OSGKeyboardExt/Views/AppContextChip.swift

@@ -0,0 +1,76 @@
+// AppContextChip.swift
+// OSGKeyboard · Keyboard Extension
+//
+// Surfaces the v0.3.0 per-app polish context on the keyboard top
+// bar. The LLM prompt already adapts to the detected context (see
+// `PolishingService.buildPrompt(for:context:)`), but without a UI
+// cue the user has no way to know "I'm currently in code mode" —
+// and no way to override the heuristic when it guesses wrong.
+//
+// Tap the chip → cycle through the five `AppContext` cases. The new
+// value is written to `AppGroupStore.setDetectedAppContext(_:at:)`,
+// so the next `PolishingService` call picks it up immediately.
+
+import SwiftUI
+import OSGKeyboardShared
+
+struct AppContextChip: View {
+    @Environment(\.themePalette) private var palette: ThemePalette
+
+    @ObservedObject var state: KeyboardViewController.State
+
+    var body: some View {
+        Menu {
+            ForEach(AppContext.allCases, id: \.self) { context in
+                Button {
+                    state.setAppContext(context)
+                } label: {
+                    if context == state.appContext {
+                        Label(menuLabel(for: context), systemImage: "checkmark")
+                    } else {
+                        Text(menuLabel(for: context))
+                    }
+                }
+            }
+        } label: {
+            label
+        }
+        .menuStyle(.button)
+        .accessibilityLabel(ExtL10n.text("keyboard.appContext.a11y"))
+        .accessibilityHint(ExtL10n.text("keyboard.appContext.a11yHint"))
+    }
+
+    private var label: some View {
+        HStack(spacing: 4) {
+            Image(systemName: iconName(for: state.appContext))
+            Text(chipText)
+            Image(systemName: "chevron.down")
+                .font(.system(size: 8, weight: .bold))
+        }
+        .font(TypeStyle.caption2)
+        .foregroundStyle(palette.textPrimary)
+        .padding(.horizontal, Spacing.xs + 2)
+        .padding(.vertical, 6)
+        .frame(minHeight: 28)
+        .background(palette.surfaceElevated, in: Capsule())
+        .overlay(Capsule().stroke(palette.divider, lineWidth: 0.5))
+    }
+
+    private var chipText: String {
+        ExtL10n.text("keyboard.appContext.chip.\(state.appContext.rawValue)")
+    }
+
+    private func menuLabel(for context: AppContext) -> String {
+        ExtL10n.text("keyboard.appContext.menu.\(context.rawValue)")
+    }
+
+    private func iconName(for context: AppContext) -> String {
+        switch context {
+        case .code:     return "chevron.left.forwardslash.chevron.right"
+        case .email:    return "envelope"
+        case .chat:     return "bubble.left"
+        case .document: return "doc.text"
+        case .unknown:  return "questionmark.circle"
+        }
+    }
+}