fix(nvim): use coc#pum API for Enter confirmation

pumvisible() checks the native Vim pum, which coc.nvim bypasses in
favour of its own popup — so the confirm never fired. Switch to
coc#pum#visible() / coc#pum#confirm() to match the Tab/S-Tab bindings.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

nvim/init.lua

@@ -140,5 +140,5 @@ end, { expr = true, silent = true })
 
 -- CR confirms CoC selection
 vim.keymap.set("i", "<CR>", function()
-    return vim.fn.pumvisible() == 1 and "<C-Y>" or "<CR>"
-end, { expr = true })
+    return vim.fn["coc#pum#visible"]() == 1 and vim.fn["coc#pum#confirm"]() or "<CR>"
+end, { expr = true, silent = true })