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/init.lua

@@ -190,7 +190,9 @@ end
 ---@param bufnr? integer
 function M.open(bufnr)
   bufnr = bufnr or vim.api.nvim_get_current_buf()
-  if not compiler.open(bufnr) then
+  local name = M.resolve_provider(bufnr)
+  local open_config = name and config.providers[name] and config.providers[name].open
+  if not compiler.open(bufnr, open_config) then
     vim.notify('[preview.nvim] no output file available for this buffer', vim.log.levels.WARN)
   end
 end