fix(kattis): fix nil display_name in contest picker (#332)

## Problem

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

## Solution

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

scrapers/kattis.py

@@ -123,7 +123,9 @@ 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