refactor(commands): move login/logout under platform namespace

Problem: `:CP login codeforces` was inconsistent with every other
command which uses `:CP <platform> <action>` ordering.

Solution: restructure to `:CP codeforces login` / `:CP codeforces
logout`. Remove `login`/`logout` from top-level `ACTIONS` and move
parsing into the platform branch of `parse_command`. Update completion
to offer `login`/`logout` as platform subcommands.

lua/cp/constants.lua

@@ -13,8 +13,6 @@ M.ACTIONS = {
   'race',
   'stress',
   'submit',
-  'login',
-  'logout',
 }
 
 M.PLATFORM_DISPLAY_NAMES = {