fix(setup): set language state before setup_code hook on first open

Problem: when opening a contest for the first time (metadata not
cached), the setup_code hook fired before state.set_language() was
called, causing state.get_language() to return nil inside the hook.

Solution: call state.set_language(lang) before the hook in the
provisional-buffer branch of setup_contest(). The value is already
computed at that point and is identical to what setup_problem() sets
later, so the early write is idempotent.

lua/cp/setup.lua

@@ -160,6 +160,8 @@ function M.setup_contest(platform, contest_id, problem_id, language)
     vim.bo[bufnr].buftype = ''
     vim.bo[bufnr].swapfile = false
 
+    state.set_language(lang)
+
     if cfg.hooks and cfg.hooks.setup_code and not vim.b[bufnr].cp_setup_done then
       local ok = pcall(cfg.hooks.setup_code, state)
       if ok then