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.

lua/cp/login.lua

@@ -0,0 +1,32 @@
+local M = {}
+
+local cache = require('cp.cache')
+local logger = require('cp.log')
+local state = require('cp.state')
+
+function M.login(platform)
+  platform = platform or state.get_platform()
+  if not platform then
+    logger.log('No platform specified. Usage: :CP login <platform>', vim.log.levels.ERROR)
+    return
+  end
+
+  vim.ui.input({ prompt = platform .. ' username: ' }, function(username)
+    if not username or username == '' then
+      logger.log('Login cancelled', vim.log.levels.WARN)
+      return
+    end
+    vim.fn.inputsave()
+    local password = vim.fn.inputsecret(platform .. ' password: ')
+    vim.fn.inputrestore()
+    if not password or password == '' then
+      logger.log('Login cancelled', vim.log.levels.WARN)
+      return
+    end
+    cache.load()
+    cache.set_credentials(platform, { username = username, password = password })
+    logger.log(platform .. ' credentials saved', vim.log.levels.INFO)
+  end)
+end
+
+return M