Linux: fix island placement on GNOME and other desktops without layer…

[mojahid2021]

This pull request introduces significant improvements to how the "island" window is positioned and displayed on Linux, especially in GNOME and other environments lacking layer-shell support. The changes ensure the island is placed just below the top panel (rather than over it), improve backend selection for correct window placement, and avoid unwanted desktop space reservation. There are also updates to documentation and versioning.

Linux window placement and backend selection:

• Added logic to select the appropriate GDK backend (wayland or x11) at startup, ensuring the island window is anchored correctly depending on compositor and layer-shell support. This includes honoring a new COUCOU_GDK_BACKEND override and falling back to X11 when layer-shell is unavailable but an X server is reachable.
• The island now sits just under the top panel (using the work area reported by the window manager) when layer-shell is not available, preventing it from obscuring system UI elements like the clock and date on GNOME. [1] [2]
• Introduced avoid_panels and top_panel_height functions to handle panel avoidance and correct vertical placement, with platform-specific no-ops for Windows. [1] [2] [3]

Documentation and configuration:

• Updated windows/README.md to clarify the island's behavior on different Linux compositors, backend selection logic, and how the window is placed relative to top panels.

Testing and maintenance:

• Added tests for X11 socket parsing to ensure correct backend selection logic.

Versioning:

• Bumped the application version from 0.1.0 to 0.1.1 in package-lock.json.