feat: add Arknights: Endfield, and stop passing off requirements as summaries

Endfield was previously written off as undatable. That was wrong: a first
pass only inspected its Duration rows (all "Permanently Available") and its
year-less version grid, and missed an "Event | Schedule & Summary" table
whose cells read "Period: 08/09/26 - 08/30/26 During the event...". Two real
events, with a year.

Adds MM/DD/YY range parsing for that shape. Month-first ordering is not
assumed lightly — Endfield's own version grid reads 01/22, 04/17, 07/16 for
versions 1.0, 1.2 and 1.4, chronological only if the month comes first.

Also stops presenting unlock conditions as descriptions. Several templates
put "Reach Union Level 8" where a blurb would go; filling the summary slot
with text that never says what the event is is worse than leaving it empty.
Wuthering Waves correctly drops to zero summaries as a result.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

src/client/state/useMarkSet.ts

@@ -0,0 +1,56 @@
+import { useCallback, useEffect, useState } from "react";
+import { readJson, writeJson } from "./storage.ts";
+
+export interface Mark {
+  /** When the reader made this mark. */
+  at: string;
+}
+export type Marks = Record<string, Mark>;
+
+/**
+ * A set of per-event marks, keyed by event ID and persisted locally.
+ *
+ * Completions ("I finished this") and ignores ("stop showing me this") are the
+ * same shape and want the same guarantees, so they share one implementation.
+ * They stay separate stores because they mean different things: an ignored
+ * event is hidden, a completed one is dimmed but still counted.
+ */
+export function useMarkSet(storageKey: string) {
+  const [marks, setMarks] = useState<Marks>(() =>
+    readJson<Marks>(storageKey, {}),
+  );
+
+  useEffect(() => {
+    writeJson(storageKey, marks);
+  }, [storageKey, marks]);
+
+  const toggle = useCallback((id: string) => {
+    setMarks((prev) => {
+      if (prev[id] !== undefined) {
+        const { [id]: _removed, ...rest } = prev;
+        return rest;
+      }
+      return { ...prev, [id]: { at: new Date().toISOString() } };
+    });
+  }, []);
+
+  const clear = useCallback(() => setMarks({}), []);
+
+  /**
+   * Union merge, keeping the earlier mark. Never removes: nothing else holds a
+   * copy of these, so a silent deletion would be unrecoverable.
+   */
+  const merge = useCallback((incoming: Marks) => {
+    setMarks((prev) => {
+      const next = { ...prev };
+      for (const [id, value] of Object.entries(incoming)) {
+        const existing = next[id];
+        next[id] =
+          existing === undefined || value.at < existing.at ? value : existing;
+      }
+      return next;
+    });
+  }, []);
+
+  return { marks, toggle, merge, clear };
+}