fix: escape on save prompt cancels select

Problem: when `prompt_save_on_select_new_entry` is enabled and the user
presses Escape on the "Save changes?" confirm dialog, `vim.fn.confirm`
returns 0, but the select continued as if the user had chosen "No".

Solution: add an explicit `choice == 0` branch that returns immediately,
aborting the select without saving or opening any files.

lua/canola/init.lua

@@ -716,6 +716,8 @@ M.select = function(opts, callback)
     local ok, choice = pcall(vim.fn.confirm, 'Save changes?', 'Yes\nNo', 1)
     if not ok then
       return finish()
+    elseif choice == 0 then
+      return
     elseif choice == 1 then
       M.save()
       return finish()