fix: handle nirilayout.kdl as a regular file (noctalia 5)

[Piero-93]

fix: crash on startup when nirilayout.kdl is a regular file (noctalia 5)

Problem

nirilayout crashes on startup with a nil pointer dereference for users on
noctalia 5:

panic: runtime error: invalid memory address or nil pointer dereference
...
gtk/v4.(*FlowBox).Insert(...)
nirilayout.Run(...) nirilayout.go:123

noctalia 5 writes ~/.config/niri/nirilayout.kdl as a regular file
(a copy of the active layout) instead of a symlink to it. os.Readlink
then fails with EINVAL, but only fs.ErrNotExist was handled, so the error
was propagated as fatal to Run. The FlowBox was never created (selector
stayed nil) while the layout loop still ran, dereferencing the nil selector.

Fix

• Add CurrentLayoutPath, which resolves the symlink when present (classic
  setup) and otherwise matches the file's contents against the known layout
  files (noctalia 5). Active-layout preselection now works in both setups,
  with no command-line flag.
• Stop treating current-layout detection as a fatal error (it only affects
  which layout is preselected).
• Guard the layout loop against a nil selector as defense in depth.

Testing

• go build ./... and go test ./... pass.
• Reproduced the original crash with a regular-file nirilayout.kdl; the
  rebuilt binary no longer crashes and shows the layout list.
• Verified the active layout is still highlighted with both a symlink and a
  regular-file nirilayout.kdl.
• Working on both shells.