header_bar: add WindowControlsPosition (macOS-style left controls)

Adds a new public enum `WindowControlsPosition { Start, End }` and a
matching field on `HeaderBar`, allowing window controls (close / minimize
/ maximize) to be packed on the start side of the headerbar (macOS
style, icon order close → minimize → maximize) instead of the default
end side (Linux / GNOME style, minimize → maximize → close).

Wiring:
- `crate::widget::WindowControlsPosition` re-exported alongside
  `HeaderBar`.
- `HeaderBar::controls_position(Option<WindowControlsPosition>)` setter;
  when left unset, falls back to `crate::config::window_controls_position()`
  (reads `CosmicTk.window_controls_position`), mirroring how `density`
  falls back to `header_size()`.
- New `CosmicTk.window_controls_position` field with default `End` for
  backwards compatibility; serde-friendly enum so existing configs keep
  working via `#[serde(default)]` semantics.

Tested with cosmic-yoterm, cosmic-settings, cosmic-edit, cosmic-files
rebuilt against this libcosmic via a local `[patch]` override. Config
changes picked up live through the existing cosmic-config subscription.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/config/mod.rs

@@ -4,6 +4,7 @@
 //! Configurations available to libcosmic applications.
 
 use crate::cosmic_theme::Density;
+use crate::widget::WindowControlsPosition;
 use cosmic_config::cosmic_config_derive::CosmicConfigEntry;
 use cosmic_config::{Config, CosmicConfigEntry};
 use serde::{Deserialize, Serialize};
@@ -67,6 +68,12 @@ pub fn header_size() -> Density {
     COSMIC_TK.read().unwrap().header_size
 }
 
+/// Position of the window control buttons (close / minimize / maximize).
+#[allow(clippy::missing_panics_doc)]
+pub fn window_controls_position() -> WindowControlsPosition {
+    COSMIC_TK.read().unwrap().window_controls_position
+}
+
 /// Interface density.
 #[allow(clippy::missing_panics_doc)]
 pub fn interface_density() -> Density {
@@ -109,6 +116,10 @@ pub struct CosmicTk {
 
     /// Mono font family
     pub monospace_font: FontConfig,
+
+    /// Side on which window control buttons (close / minimize / maximize)
+    /// are placed. `End` = right (Linux / GNOME), `Start` = left (macOS).
+    pub window_controls_position: WindowControlsPosition,
 }
 
 impl Default for CosmicTk {
@@ -132,6 +143,7 @@ impl Default for CosmicTk {
                 stretch: iced::font::Stretch::Normal,
                 style: iced::font::Style::Normal,
             },
+            window_controls_position: WindowControlsPosition::default(),
         }
     }
 }