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.

docs/APPSTORE_METADATA.md

@@ -38,6 +38,7 @@
 | Field | Value |
 |---|---|
 | **Price** | Free (0 USD) |
+| **In-App Purchases** | Optional voluntary tip — Consumable `ByRockyACoffee` (¥28 China tier; no feature unlock) |
 | **Availability** | All App Store territories (default) |
 | **Pre-order** | No |
 | **Volume purchase** | No |
@@ -83,6 +84,12 @@ WHY OSGKEYBOARD
   touches (voice audio + transcripts, on-device by default, never
   linked or tracked); we don't run a server.
 
+OPTIONAL SUPPORT
+
+OSGKeyboard is completely free — every feature is available without
+payment. If you'd like to support development, Settings includes an
+optional in-app tip (Consumable). It does not unlock anything extra.
+
 BUILT FOR
 
 • iOS 26 and later, iPhone and iPad.
@@ -271,6 +278,11 @@ to end, please:
 5. The privacy policy is at
    https://hkgood.github.io/OSGKeyboard/privacy/
 
+6. Optional tip (Consumable IAP ByRockyACoffee): open
+   Settings → "Support the Developer". All features remain free before
+   and after purchase; the tip does not unlock anything. Consumable
+   tips cannot be restored (stated in UI).
+
 Source code: https://github.com/hkgood/OSGKeyboard
 ```