feat(commands): standalone :CP login/logout with auto-restore

Problem: `:CP login` and `:CP logout` only worked as `:CP <platform>
login`. Users with an active platform or in a registered buffer had to
type the platform name redundantly.

Solution: Parse standalone `login`/`logout`/`signup` as action commands
with `requires_context = true`, resolving the platform from state (or
auto-restoring from the current buffer). Add `vim.cmd.redraw()` after
auto-restore so the screen updates before any prompt. Add `login`/
`logout` to top-level completion when a platform is active.

plugin/cp.lua

@@ -43,6 +43,10 @@ end, {
       vim.list_extend(candidates, platforms)
       table.insert(candidates, 'cache')
       table.insert(candidates, 'pick')
+      if platform then
+        table.insert(candidates, 'login')
+        table.insert(candidates, 'logout')
+      end
       if platform and contest_id then
         vim.list_extend(
           candidates,