title_bar: Skip drawn window controls under server-side decorations (#2773)

## Description

`WindowControls` draws min/max/close on every non-macOS platform
**unconditionally**. When a window ends up with **server-side**
decorations, the window manager renders its own title bar as well, so
two sets of controls stack on top of each other — most visibly **two
close buttons**.

This is not a corner case; gpui ends up with server-side decorations in
common setups:

- **X11 without a compositor** (e.g. a default XFCE session):
`request_decorations(Client)` falls back to server-side because
client-side decorations need compositor support.
- **Wayland**: a compositor may grant **server mode** via xdg-decoration
even when client mode was requested — `set_mode` is only a preference.

The title bar already gates its client-side window-menu overlay on
`is_client_decorated` (title_bar.rs:320); this PR applies the same
gating to the drawn controls, so exactly **one** set of window controls
is visible in every combination:

| Decorations | Who shows min/max/close |
| --- | --- |
| Client-side (Windows, GNOME Wayland fallback, X11 with compositor) |
our drawn controls |
| Server-side | the WM's controls (we draw nothing) |

The guard is `cfg(target_os = "linux")`; Windows and macOS behavior is
unchanged.

> 🤖 **AI assistance:** the patch and this description were drafted with
AI assistance and reviewed by a human. The same change is deployed in a
downstream app built on gpui-component where the duplicated controls
were first reported.

## Screenshot

Linux captures pending — the duplication needs a Linux session to
reproduce (X11 without a compositor, or KWin with server-side
decorations). Happy to attach before/after screenshots of a story if
maintainers want them; the downstream app confirmed the behavior.

## How to Test

1. `cargo run` the story book and open any story rendering a `TitleBar`.
2. **X11, no compositor** (XFCE with compositing disabled, or a bare
`startx` session): before this PR the window shows the WM title bar
*plus* our drawn min/max/close; after, only the WM's controls.
3. **Wayland with server-side decorations** (e.g. KWin CSD disabled):
same duplication before, single set after.
4. **macOS / Windows**: no change (guard is Linux-only).

## Checklist

- [x] I have read the [CONTRIBUTING](../CONTRIBUTING.md) document and
followed the guidelines.
- [x] Reviewed the changes in this PR and confirmed AI generated code
(If any) is accurate.
- [ ] Passed `cargo run` for story tests related to the changes.
- [x] Tested macOS, Windows and Linux platforms performance (if the
change is platform-specific) — macOS verified unchanged; Linux verified
in the downstream app carrying the identical patch; Windows unaffected
by construction.