feat: tip IAP, commercial GitHub Pages, and App Store screenshots

Add optional StoreKit support tip (features stay free), redesign the
landing page with zh/en + light/dark and product shots, and ship
iPhone/iPad/Mac App Store screenshot sets.

OSGKeyboardTests/TipProductTests.swift

@@ -0,0 +1,39 @@
+// TipProductTests.swift
+// OSGKeyboardTests
+//
+// Locks tip product identifiers and optional support-count persistence.
+
+import XCTest
+@testable import OSGKeyboardShared
+
+final class TipProductTests: XCTestCase {
+    private var suiteName: String!
+    private var defaults: UserDefaults!
+
+    override func setUp() {
+        super.setUp()
+        suiteName = "tip.tests.\(UUID().uuidString)"
+        defaults = UserDefaults(suiteName: suiteName)!
+        defaults.removePersistentDomain(forName: suiteName)
+    }
+
+    override func tearDown() {
+        defaults.removePersistentDomain(forName: suiteName)
+        defaults = nil
+        super.tearDown()
+    }
+
+    func testSupportProductIDIsStable() {
+        XCTAssertEqual(TipProduct.supportID, "ByRockyACoffee")
+        XCTAssertEqual(TipProduct.allProductIDs, [TipProduct.supportID])
+    }
+
+    func testSupportCountDefaultsKeyIsStable() {
+        XCTAssertEqual(TipProduct.supportCountDefaultsKey, "tipSupportPurchaseCount")
+    }
+
+    func testSupportCountPersistsInUserDefaults() {
+        defaults.set(2, forKey: TipProduct.supportCountDefaultsKey)
+        XCTAssertEqual(defaults.integer(forKey: TipProduct.supportCountDefaultsKey), 2)
+    }
+}