fix(commands): register VimLeavePre to call stop_all

Problem: spawned compiler processes and watching autocmds were never
cleaned up when Neovim exited, leaving orphaned processes running.

Solution: register a VimLeavePre autocmd in commands setup that calls
compiler.stop_all(), which kills active processes, unwatches all
buffers, and stops the reload server.

lua/preview/commands.lua

@@ -57,6 +57,12 @@ function M.setup()
     end,
     desc = 'Toggle, compile, clean, open, or check status of document preview',
   })
+
+  vim.api.nvim_create_autocmd('VimLeavePre', {
+    callback = function()
+      require('preview.compiler').stop_all()
+    end,
+  })
 end
 
 return M