fix(clipboard): copied templates vanish on Wayland

[LashaKamadadze]

Problem

On Wayland compositors (I tested on Hyprland), the clipboard actions in
search (Shift+C, Shift+X) print "copied to clipboard," but nothing
ends up there. wl-paste comes back empty and no Wayland app can
paste it.

Root cause

copypasta always picks its X11 backend on Linux. From src/lib.rs:

#[cfg(feature = "x11")]
pub type ClipboardContext = x11_clipboard::X11ClipboardContext;

No runtime check for WAYLAND_DISPLAY, no fallback. If the x11
feature is on (it is by default, alongside wayland), ClipboardContext
is the X11 type, full stop. Reaching copypasta's Wayland backend means
calling wayland_clipboard::create_clipboards_from_external(display_ptr)
yourself with a raw Wayland display handle. gitnr never does that.

This isn't a gap copypasta is unaware of. The maintainer has said as
much directly:

• Is Wayland supported by default, or is additional logic required for this? alacritty/copypasta#27 - "It's impossible
  to do this automatically. You require a display handle from your
  window to access the clipboard on Wayland."
• Copying to clipboard not working under Wayland/Hyprland alacritty/copypasta#71 - a Smithay/winit
  maintainer telling users "this crate requires a running window...
  If you don't show any window and want a clipboard manager, use
  wl-clipboard-rs."

gitnr is exactly that second case, a TUI with no window at all. So on
Wayland it opens XWayland and sets clipboard ownership on a headless,
invisible X11 window. The call succeeds, hence "Success," but the X11
selection only lives as long as the owning process answers paste
requests, and compositor XWayland-to-Wayland clipboard bridging is
generally keyed to visible or focused Xwayland surfaces. A headless
clipboard-only window often gets skipped by that bridge. The copy is
gone the moment gitnr exits, sometimes sooner.

Fix

Switched to arboard. On Linux it checks WAYLAND_DISPLAY at runtime
and, when set, uses the wlr-data-control protocol, the same one
wl-copy and cliphist use (and, notably, the same wl-clipboard-rs the
copypasta maintainer pointed people to). That protocol works with no
window, which is what a TUI needs. It falls back to X11 automatically
when Wayland isn't there.

I also had to turn on the wayland-data-control feature by hand.
arboard's default feature is image-data, not Wayland support. I
turned image-data off too, since gitnr only ever copies text. That
drops the image crate and its PNG decoding stack from the dependency
tree.

Testing

• cargo test: 21 passed
• cargo clippy --all-targets: no new warnings
• cargo build --release: clean
• Tested by hand on Hyprland: selected templates in search, copied
  with Shift+C, checked wl-paste and pasted into another app.
  Previously produced nothing.