window-management: toggle for macOS-style window controls position

Adds a new toggler "Place buttons on the left (macOS style)" in
*Desktop → Window Management → Window controls*, next to the existing
show_maximize / show_minimize controls. When enabled, writes
`WindowControlsPosition::Start` into the `CosmicTk` config
(com.system76.CosmicTk); when disabled, writes `End` (default).

All libcosmic-based applications rebuilt against a libcosmic that
exposes `WindowControlsPosition` pick up the change live through the
existing cosmic-config subscription.

Includes:
- new Message variant WindowControlsPositionStart(bool) + handler
- `window-controls.position-start` fluent key added in en + fr locales
- Cargo.toml: activate local [patch] override to libcosmic (dev only,
  not suitable for upstream merge)

Requires: libcosmic >= 5c331935 (header_bar WindowControlsPosition).

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

i18n/en/cosmic_settings.ftl

@@ -484,6 +484,7 @@ window-controls = Window controls
     .maximize = Show maximize button
     .minimize = Show minimize button
     .active-window-hint = Show active window hint
+    .position-start = Place buttons on the left (macOS style)
 
 focus-navigation = Focus navigation
     .focus-follows-cursor = Focus follows cursor