Timeout stuck at 0:01 and jagged rounded corners on Windows

[TsekNet]

Problem

1. Timeout countdown stuck at 0:01: The frontend sends timeout:<value> but Respond() validates the raw string including the prefix, rejecting it as unknown.
2. Jagged rounded corners on Windows: SetWindowRgn with CreateRoundRectRgn produces aliased (staircase) edges with no anti-aliasing.

Fix

1. Strip the timeout: prefix before validation in Respond().
2. Layered rounding: CSS border-radius on a .window-frame wrapper for smooth anti-aliased visual corners, black body margin for contrast, SetWindowRgn to clip the outer rectangle.

Ref #23