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/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