fix(kattis): fix nil display_name in contest picker

Problem: `ContestSummary.display_name` defaults to `None`, which
serializes to JSON `null` → Lua `vim.NIL`. The picker displayed
"vim.NIL" for every entry, and `start_time` was always stored
because `vim.NIL` is truthy in Lua.

Solution: pass `display_name=name` explicitly in `_parse_contests_page`
so JSON never emits `null`. In `set_contest_summaries`, coerce
`display_name` via a `~= vim.NIL` guard and apply the same guard
before storing `start_time`.

lua/cp/cache.lua

@@ -396,9 +396,9 @@ function M.set_contest_summaries(platform, contests)
   cache_data[platform] = cache_data[platform] or {}
   for _, contest in ipairs(contests) do
     cache_data[platform][contest.id] = cache_data[platform][contest.id] or {}
-    cache_data[platform][contest.id].display_name = contest.display_name
+    cache_data[platform][contest.id].display_name = (contest.display_name ~= vim.NIL and contest.display_name) or contest.name
     cache_data[platform][contest.id].name = contest.name
-    if contest.start_time then
+    if contest.start_time and contest.start_time ~= vim.NIL then
       cache_data[platform][contest.id].start_time = contest.start_time
     end
   end

scrapers/kattis.py

@@ -123,7 +123,7 @@ def _parse_contests_page(html: str) -> list[ContestSummary]:
                     start_time = int(dt.timestamp())
                 except Exception:
                     pass
-        results.append(ContestSummary(id=cid, name=name, start_time=start_time))
+        results.append(ContestSummary(id=cid, name=name, display_name=name, start_time=start_time))
     return results