linux: improve Linux client-side decorations (#2610)

## Description

Improve Linux client-side window decorations so application windows have
clearer, more native-looking separation from their surroundings.

- Increase the transparent inset available for the drop shadow.
- Replace the hard single-layer shadow with a soft GNOME-style ambient
and contact shadow.
- Reduce shadow and border strength for inactive windows.
- Use a subtle dark 1px border in light themes and a subtle light 1px
border in dark themes.
- Keep the generic wrapper square because GPUI content masks currently
clip children rectangularly; a rounded frame would expose child
backgrounds at the corners.

## How to Test

1. Run the story application on Linux with client-side decorations.
2. Check a floating window in both light and dark themes.
3. Verify the shadow fades smoothly without obvious clipping and the 1px
edge remains subtle.
4. Focus and unfocus the window and verify inactive decoration contrast
is reduced.
5. Maximize or tile the window and verify shadow and borders are removed
from tiled edges.

Validation run:

- `cargo check -p gpui-component`
- `git diff --check`

## Checklist

- [x] I have read the CONTRIBUTING document and followed the guidelines.
- [x] Reviewed the changes in this PR and confirmed AI generated code is
accurate.
- [ ] Passed `cargo run` for story tests related to the changes.
- [ ] Tested macOS, Windows and Linux platforms performance
(Linux-specific visual change).