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.

Tests/suite-manifest.json

@@ -32,7 +32,9 @@
         "OSGKeyboardTests/AccountSignInCoordinatorTests",
         "OSGKeyboardTests/AccountSnapshotLoaderTests",
         "OSGKeyboardTests/DeviceIntegrityTests",
-        "OSGKeyboardTests/ReferralProfileTests"
+        "OSGKeyboardTests/ReferralProfileTests",
+        "OSGKeyboardTests/InvitePosterTests",
+        "OSGKeyboardTests/ScreenBrightnessBoostTests"
       ]
     },
     "managed_gateway": {