fix(report): square the group card's bottom corners

Neither precedent rounds a group header the way ReportSummaryCard's
default 3px radius did. The import preview's group row has no radius at
all, and the cuesheet's rounds only the top-left, leaving the bottom
square specifically so its rail runs straight into the row below.

The report's group card sits directly on top of its own event rows with
no gap between them, so rounding all four corners curved the rail away
right at that seam — a visible break between the heading and its first
event, seen in the screenshots.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019nr3FbLbM8gB8Jm771YgTV

apps/client/src/features/app-settings/panel/feature-panel/composite/ReportTable.module.scss

@@ -45,6 +45,10 @@ td.under {
   --card-surface: color-mix(in srgb, #{$gray-1300} 88%, #{$rail} 12%);
 
   box-shadow: inset 4px 0 $rail;
+  // the card sits directly on top of its own event rows with no gap between
+  // them, so the bottom corners stay square: rounding them curved the rail
+  // away right where it needs to run straight into the row below
+  border-radius: 3px 3px 0 0;
 }
 
 .eventCue,