feat: add :CP login command for explicit credential management

Problem: credentials were only set implicitly on first :CP submit.
There was no way to update wrong credentials, log out, or set
credentials ahead of time without editing the cache JSON manually.

Solution: add :CP login [platform] which always prompts for username
and password and overwrites any saved credentials for that platform.
Omitting the platform falls back to the active platform. Wire the
command through constants, parse_command, handle_command, and add
tab-completion (suggests platform names). Document in vimdoc under
the SUBMIT section and in the commands reference.

plugin/cp.lua

@@ -103,6 +103,8 @@ end, {
           end
         end
         return filter_candidates(candidates)
+      elseif args[2] == 'login' then
+        return filter_candidates(platforms)
       elseif args[2] == 'race' then
         local candidates = { 'stop' }
         vim.list_extend(candidates, platforms)