fix: decouple python env setup from config init

Problem: setup_python_env() is called from check_required_runtime()
during config.setup(), which runs on the very first :CP command. The
uv sync and nix build calls use vim.system():wait(), blocking the
Neovim event loop. During the block the UI is frozen and
vim.schedule-based log messages never render, so the user sees an
unresponsive editor with no feedback.

Solution: remove setup_python_env() from check_required_runtime() so
config init is instant. Call it lazily from run_scraper() instead,
only when a scraper subprocess is actually needed. Use vim.notify +
vim.cmd.redraw() before blocking calls so the notification renders
immediately via a forced screen repaint, rather than being queued
behind vim.schedule.

lua/cp/scraper.lua

@@ -25,6 +25,15 @@ end
 ---@param args string[]
 ---@param opts { sync?: boolean, ndjson?: boolean, on_event?: fun(ev: table), on_exit?: fun(result: table) }
 local function run_scraper(platform, subcommand, args, opts)
+  if not utils.setup_python_env() then
+    local msg = 'no Python environment available (install uv or nix)'
+    logger.log(msg, vim.log.levels.ERROR)
+    if opts and opts.on_exit then
+      opts.on_exit({ success = false, error = msg })
+    end
+    return { success = false, error = msg }
+  end
+
   local plugin_path = utils.get_plugin_path()
   local cmd = utils.get_python_cmd(platform, plugin_path)
   vim.list_extend(cmd, { subcommand })