feat: macOS architecture, cloud ASR/LLM providers, and 6-step iOS onboarding

- Add macOS menu-bar dictation app with local ASR models (SenseVoice/Qwen3),
  global Option hotkey, and bottom overlay
- Add cloud ASR/LLM providers (Anthropic, Volcengine, Bailian, and more) with
  provider logos, model listing, and connection checks
- Add shared 7-day usage stats UI (UsageStatsCluster / SevenDayUsageChart)
- Add iOS onboarding step 6 for polish LLM setup; hide custom-language-model
  diagnostic toggle behind DEBUG
- Unify iOS onboarding tagline with the macOS brand line ("开口即文字。")
- Rewrite README (Chinese-first, product-oriented) and refresh GitHub Pages

OSGKeyboardShared/DesignSystem/UsageSurfaceCard.swift

@@ -0,0 +1,36 @@
+// UsageSurfaceCard.swift
+// OSGKeyboard · Shared
+//
+// Flat semantic surface used by home / dashboard stats on every platform.
+// Deliberately shadowless — hierarchy comes from fill + hairline border.
+
+import SwiftUI
+
+public struct UsageSurfaceCard<Content: View>: View {
+    @Environment(\.themePalette) private var palette
+
+    public var padding: CGFloat
+    public var cornerRadius: CGFloat
+    @ViewBuilder public var content: () -> Content
+
+    public init(
+        padding: CGFloat = Spacing.md,
+        cornerRadius: CGFloat = Radius.medium,
+        @ViewBuilder content: @escaping () -> Content
+    ) {
+        self.padding = padding
+        self.cornerRadius = cornerRadius
+        self.content = content
+    }
+
+    public var body: some View {
+        let shape = RoundedRectangle(cornerRadius: cornerRadius, style: .continuous)
+
+        content()
+            .padding(padding)
+            .background(palette.surface, in: shape)
+            .overlay(
+                shape.stroke(palette.divider, lineWidth: 0.5)
+            )
+    }
+}