fix(VTable): clip sticky header to container border-radius (Fixes #21394)

[waterWang]

Description

When a table has border-radius (e.g. inside a card) and fixed-header is enabled, the sticky thead header renders with a solid background that covers the container's rounded top corners, breaking the visual border-radius.

Root cause

The .v-table__wrapper has overflow: auto and border-radius: inherit, but the sticky thead inside it is positioned at top: 0 with z-index: 2. The header cells have background: $table-background which paints over the wrapper's rounded corners.

Fix

Added overflow: hidden to .v-table (the flex container), which clips the sticky header to the table's border-radius. This is a standard CSS pattern for rounded corners on scrollable containers with sticky headers.

Fixes #21394