feat(clipboard): implement proper Linux file reference clipboard support

[gregpriday]

Summary

Replaces the plain-text file:// URI fallback on Linux with proper clipboard file reference support using xclip (X11) and wl-copy (Wayland) with the correct MIME types recognised by GTK and KDE file managers.

Closes #102

Changes Made

• Add _detectLinuxClipboardMethod() static helper that detects display protocol ($WAYLAND_DISPLAY / $DISPLAY) and desktop environment ($XDG_CURRENT_DESKTOP) and returns the correct { tool, args, payload } object
• GTK-based desktops (GNOME, XFCE, Cinnamon, MATE, Budgie) and unknown DEs use x-special/gnome-copied-files MIME type with copy\nfile:///path payload
• KDE desktops use text/uri-list MIME type with file:///path\r\n payload (RFC 2483 compliant)
• X11 sessions use xclip -selection clipboard -t <mime>, Wayland sessions use wl-copy --type <mime>; Wayland takes priority when both $WAYLAND_DISPLAY and $DISPLAY are set
• Add 2-second spawnSync timeout to prevent hanging on broken Wayland compositors
• Trim env var checks to reject whitespace-only values
• Graceful fallback to plain-text file:// URI copy when neither tool is installed or the spawn fails — no error shown to user
• Expand unit tests from 1 to 44 tests covering all X11/Wayland × GTK/KDE combinations, synchronous throw path, URI encoding with spaces, case-insensitive DE detection, whitespace env var rejection