feat(account): add QR invitation poster and 8-character invitation codes

- Invite opens an appearance-matched poster with the app icon, inviter name,
  QR code, and short code; tap for a full-brightness QR view; save (add-only
  Photos access), share, or copy the link. Mac shows the poster in a popover
  and can save it as PNG.
- Accounts get look-alike-free 8-character codes. "I have a code" accepts a
  code, link, or pasted message during the server-defined binding window.
- Permanently rejected redemptions show a specific message and drop the
  stored code instead of retrying on every sign-in.

OSGKeyboardShared/DesignSystem/ColorLibrary.swift

@@ -17,6 +17,13 @@ public enum OSGColor {
     public static let fixedLightContent = Color.white
     public static let fixedDarkContent = Color.black
 
+    // MARK: - Invitation poster
+
+    /// Shared artwork must look identical for every recipient, so these do not
+    /// follow the app appearance. The background matches the app icon.
+    public static let invitePosterBackground = color(0x252527)
+    public static let invitePosterHint = color(0xA1A1A6)
+
     // MARK: - iOS app theme
 
     public static let darkBackground = color(0x0A0A0B)