On Linux with client-side decorations, opening the workspace sidebar
squared off the window corners on the sidebar's side while the rest of
the window stayed rounded:

Before:
<img width="366" height="239" alt="image"
src="https://github.com/user-attachments/assets/e0ee8d28-6143-42ea-bfd2-10d069df5492"
/>

After:
<img width="426" height="219" alt="image"
src="https://github.com/user-attachments/assets/51d2979d-0103-4742-90bd-a97f5837d99b"
/>

Previously, `client_side_decorations` took a `border_radius_tiling`
override that `MultiWorkspace` used to deliberately square the window
shape on the sidebar's side, since the sidebar painted a square
background that would otherwise poke out past the rounded window border.
The title bar and status bar already skip rounding their corners on the
sidebar side, implying the sidebar is expected to own those window
corners — it just never rounded them.

This PR makes the sidebar round its outer corners the same way the title
bar and status bar do (including overlapping the 1px window border on
untiled edges to avoid a transparent gap in the rounded corners), and
removes the now-unneeded `border_radius_tiling` parameter so the window
border and backdrop round purely based on actual tiling state.

Only applies when the window uses client-side decorations, so macOS and
Windows are unaffected.

Fixes #54724 

Release Notes:

- Fixed square window corners on the sidebar side of client-decorated
(Linux) windows when the workspace sidebar is open.

---------

Co-authored-by: Danilo Leal <67129314+danilo-leal@users.noreply.github.com>