enhance: render Others slice with secondary foreground color

[JC-Chung]

Changes

• Render the Others slice with secondaryForeground (Brush.FG2) instead of the next palette color. When the number of authors above 1% is a multiple of 10, the palette index wraps around to 0, making Others the same Orange as the adjacent top-author slice — the two blend together with no visible boundary.
• Draw a 1px separator line at the top of the pie (the head-to-tail seam) via a new BorderBrush property bound to Brush.Window. Without it, when all authors are above 1% and the count is 11/21/31..., the last author's slice is also Orange and adjacent to the top author's slice, which the Others fix cannot cover.
• Move var brush = s_brushes[brushIndex] into the percent > 0.01 branch.
• Rename _lastHoverred to _lastHovered (no behavior change).