feat(config): merge platform config model and add disabled-platform guard

Problem: Supplying any `platforms` table silently dropped all unlisted
platforms, making it easy to accidentally disable platforms. Disabled
platforms also produced no user-facing error on invocation.

Solution: Switch to a merge model — all six platforms are enabled by
default and user entries are deep-merged on top. Set a platform key to
`false` to disable it explicitly. Add a `check_platform_enabled` guard
in `handle_command` for contest fetch, login, logout, and race actions.

lua/cp/commands/init.lua

@@ -283,7 +283,7 @@ local function parse_command(args)
         message = 'Too few arguments - specify a contest.',
       }
     elseif #args == 2 then
-      if args[2] == 'login' or args[2] == 'logout' then
+      if args[2] == 'login' or args[2] == 'logout' or args[2] == 'signup' then
         return { type = 'action', action = args[2], requires_context = false, platform = first }
       end
       local contest = args[2]
@@ -330,6 +330,22 @@ local function parse_command(args)
   return { type = 'error', message = 'Unknown command or no contest context.' }
 end
 
+---@param platform string
+---@return boolean
+local function check_platform_enabled(platform)
+  local cfg = require('cp.config').get_config()
+  if not cfg.platforms[platform] then
+    logger.log(
+      ("Platform '%s' is not enabled. Add it to vim.g.cp.platforms to enable it."):format(
+        constants.PLATFORM_DISPLAY_NAMES[platform] or platform
+      ),
+      { level = vim.log.levels.ERROR }
+    )
+    return false
+  end
+  return true
+end
+
 --- Core logic for handling `:CP ...` commands
 ---@return nil
 function M.handle_command(opts)
@@ -378,6 +394,9 @@ function M.handle_command(opts)
     elseif cmd.action == 'submit' then
       require('cp.submit').submit({ language = cmd.language })
     elseif cmd.action == 'race' then
+      if not check_platform_enabled(cmd.platform) then
+        return
+      end
       require('cp.race').start(cmd.platform, cmd.contest, cmd.language)
     elseif cmd.action == 'race_stop' then
       require('cp.race').stop()
@@ -396,9 +415,25 @@ function M.handle_command(opts)
       end
       vim.ui.open(url)
     elseif cmd.action == 'login' then
+      if not check_platform_enabled(cmd.platform) then
+        return
+      end
       require('cp.credentials').login(cmd.platform)
     elseif cmd.action == 'logout' then
+      if not check_platform_enabled(cmd.platform) then
+        return
+      end
       require('cp.credentials').logout(cmd.platform)
+    elseif cmd.action == 'signup' then
+      local url = constants.SIGNUP_URLS[cmd.platform]
+      if not url then
+        logger.log(
+          ("No signup URL available for '%s'"):format(cmd.platform),
+          { level = vim.log.levels.WARN }
+        )
+        return
+      end
+      vim.ui.open(url)
     end
   elseif cmd.type == 'problem_jump' then
     local platform = state.get_platform()
@@ -432,6 +467,9 @@ function M.handle_command(opts)
     local cache_commands = require('cp.commands.cache')
     cache_commands.handle_cache_command(cmd)
   elseif cmd.type == 'contest_setup' then
+    if not check_platform_enabled(cmd.platform) then
+      return
+    end
     local setup = require('cp.setup')
     setup.setup_contest(cmd.platform, cmd.contest, nil, cmd.language)
     return

lua/cp/config.lua

@@ -107,6 +107,7 @@
 ---@field runtime { effective: table<string, table<string, CpLanguage>> }  -- computed
 
 ---@class cp.PartialConfig: cp.Config
+---@field platforms? table<string, CpPlatform|false>
 
 local M = {}
 
@@ -333,13 +334,18 @@ function M.setup(user_config)
   vim.validate({ user_config = { user_config, { 'table', 'nil' }, true } })
   local defaults = vim.deepcopy(M.defaults)
   if user_config and user_config.platforms then
-    for plat in pairs(defaults.platforms) do
-      if not user_config.platforms[plat] then
+    for plat, v in pairs(user_config.platforms) do
+      if v == false then
         defaults.platforms[plat] = nil
       end
     end
   end
   local cfg = vim.tbl_deep_extend('force', defaults, user_config or {})
+  for plat, v in pairs(cfg.platforms) do
+    if v == false then
+      cfg.platforms[plat] = nil
+    end
+  end
 
   if not next(cfg.languages) then
     error('[cp.nvim] At least one language must be configured')