fix(compiler): respect provider open config in M.open()

Problem: :Preview open always called vim.ui.open regardless of the
provider's open field. Providers configured with a custom opener
(e.g. open = { 'sioyek', '--new-instance' }) were ignored, so the
first build opened with sioyek but :Preview open fell back to the
system default.

Solution: init.lua resolves the provider's open config and passes it
to compiler.open(). If open_config is a table, the custom command is
spawned with the output path appended. Otherwise vim.ui.open is used
as before.

lua/preview/compiler.lua

@@ -443,13 +443,19 @@ end
 
 ---@param bufnr integer
 ---@return boolean
-function M.open(bufnr)
+function M.open(bufnr, open_config)
   local output = last_output[bufnr]
   if not output then
     log.dbg('no last output file for buffer %d', bufnr)
     return false
   end
-  vim.ui.open(output)
+  if type(open_config) == 'table' then
+    local open_cmd = vim.list_extend({}, open_config)
+    table.insert(open_cmd, output)
+    vim.system(open_cmd)
+  else
+    vim.ui.open(output)
+  end
   return true
 end