add autocommand for theme update on SIGUSR1 signal

This change sets up an autocommand to update the colorscheme
when the system theme changes, triggered by the SIGUSR1 signal
from an external watcher script.

lua/settings.lua

@@ -23,6 +23,16 @@ end
 -- Set background based on system theme on startup
 set_background_from_system()
 
+-- Handle SIGUSR1 signal to update theme when system theme changes
+-- External watcher script (scripts/theme-watcher.sh) sends this signal
+vim.api.nvim_create_autocmd('Signal', {
+  pattern = 'SIGUSR1',
+  callback = function()
+    set_background_from_system()
+  end,
+  desc = 'Update colorscheme when system theme changes',
+})
+
 -- Store initial working directory as project root
 -- Used by netrw navigation to return to project root after following symlinks
 vim.g.project_root = vim.fn.getcwd()