feat(commands): implement :CP open [problem|contest|standings]

Problem: There was no way to open a problem or contest page in the
browser from within the plugin; users had to manually navigate to
the platform URL.

Solution: Add `contest_url` and `standings_url` to `MetadataResult`
and persist them via `cache.set_contest_data`. Add `cache.get_open_urls`
to assemble all three URLs from cache. Wire up `:CP open` in
`commands/init.lua` to call `vim.ui.open` on the resolved URL,
warning when none is available (e.g. CSES standings).

lua/cp/cache.lua

@@ -10,6 +10,8 @@
 ---@field name string
 ---@field display_name string
 ---@field url string
+---@field contest_url string
+---@field standings_url string
 
 ---@class ContestSummary
 ---@field display_name string
@@ -148,12 +150,16 @@ end
 ---@param contest_id string
 ---@param problems Problem[]
 ---@param url string
-function M.set_contest_data(platform, contest_id, problems, url)
+---@param contest_url string
+---@param standings_url string
+function M.set_contest_data(platform, contest_id, problems, url, contest_url, standings_url)
   vim.validate({
     platform = { platform, 'string' },
     contest_id = { contest_id, 'string' },
     problems = { problems, 'table' },
     url = { url, 'string' },
+    contest_url = { contest_url, 'string' },
+    standings_url = { standings_url, 'string' },
   })
 
   cache_data[platform] = cache_data[platform] or {}
@@ -165,6 +171,8 @@ function M.set_contest_data(platform, contest_id, problems, url)
     problems = problems,
     index_map = {},
     url = url,
+    contest_url = contest_url,
+    standings_url = standings_url,
   }
   for i, p in ipairs(out.problems) do
     out.index_map[p.id] = i
@@ -174,6 +182,25 @@ function M.set_contest_data(platform, contest_id, problems, url)
   M.save()
 end
 
+---@param platform string?
+---@param contest_id string?
+---@param problem_id string?
+---@return { problem: string|nil, contest: string|nil, standings: string|nil }|nil
+function M.get_open_urls(platform, contest_id, problem_id)
+  if not platform or not contest_id then
+    return nil
+  end
+  if not cache_data[platform] or not cache_data[platform][contest_id] then
+    return nil
+  end
+  local cd = cache_data[platform][contest_id]
+  return {
+    problem = cd.url ~= '' and problem_id and string.format(cd.url, problem_id) or nil,
+    contest = cd.contest_url ~= '' and cd.contest_url or nil,
+    standings = cd.standings_url ~= '' and cd.standings_url or nil,
+  }
+end
+
 ---@param platform string
 ---@param contest_id string
 function M.clear_contest_data(platform, contest_id)

lua/cp/commands/init.lua

@@ -245,6 +245,12 @@ local function parse_command(args)
         debug = debug,
         mode = mode,
       }
+    elseif first == 'open' then
+      local target = args[2] or 'problem'
+      if not vim.tbl_contains({ 'problem', 'contest', 'standings' }, target) then
+        return { type = 'error', message = 'Usage: :CP open [problem|contest|standings]' }
+      end
+      return { type = 'action', action = 'open', requires_context = true, subcommand = target }
     elseif first == 'pick' then
       local language = nil
       if #args >= 3 and args[2] == '--lang' then
@@ -375,6 +381,20 @@ function M.handle_command(opts)
       require('cp.race').start(cmd.platform, cmd.contest, cmd.language)
     elseif cmd.action == 'race_stop' then
       require('cp.race').stop()
+    elseif cmd.action == 'open' then
+      local cache = require('cp.cache')
+      cache.load()
+      local urls =
+        cache.get_open_urls(state.get_platform(), state.get_contest_id(), state.get_problem_id())
+      local url = urls and urls[cmd.subcommand]
+      if not url or url == '' then
+        logger.log(
+          ("No URL available for '%s'"):format(cmd.subcommand),
+          { level = vim.log.levels.WARN }
+        )
+        return
+      end
+      vim.ui.open(url)
     elseif cmd.action == 'login' then
       require('cp.credentials').login(cmd.platform)
     elseif cmd.action == 'logout' then

lua/cp/constants.lua

@@ -13,6 +13,7 @@ M.ACTIONS = {
   'race',
   'stress',
   'submit',
+  'open',
 }
 
 M.PLATFORM_DISPLAY_NAMES = {

lua/cp/setup.lua

@@ -243,7 +243,14 @@ function M.setup_contest(platform, contest_id, problem_id, language)
       contest_id,
       vim.schedule_wrap(function(result)
         local problems = result.problems or {}
-        cache.set_contest_data(platform, contest_id, problems, result.url)
+        cache.set_contest_data(
+          platform,
+          contest_id,
+          problems,
+          result.url,
+          result.contest_url or '',
+          result.standings_url or ''
+        )
         local prov = state.get_provisional()
         if not prov or prov.platform ~= platform or prov.contest_id ~= contest_id then
           return