feat: add local-only completion and preference state

Completions and prefs live in localStorage and never reach the server —
there is no account and nothing to log into. Reads never throw, so a corrupt
value costs a preference rather than the whole screen.

Import merges and never removes: a completion present on either side stays
completed, because nothing else holds a copy to restore from.

The feed client refuses a schemaVersion it does not know rather than
guessing at unfamiliar fields.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/client/api.ts

@@ -0,0 +1,34 @@
+import { EventFeed, SCHEMA_VERSION } from "../shared/feed.ts";
+
+export type FeedState =
+  | { status: "loading" }
+  | { status: "ready"; feed: EventFeed }
+  | { status: "error"; message: string };
+
+/**
+ * Fetch the published feed.
+ *
+ * A `schemaVersion` we do not recognise is refused rather than rendered — the
+ * client would be guessing at unfamiliar fields, and a calendar that guesses is
+ * worse than one that asks you to reload.
+ */
+export async function fetchFeed(signal?: AbortSignal): Promise<EventFeed> {
+  const res = await fetch("/data/events.v1.json", { signal: signal ?? null });
+  if (!res.ok) {
+    throw new Error(`Feed request failed (${res.status}).`);
+  }
+
+  const json: unknown = await res.json();
+  const version = (json as { schemaVersion?: number }).schemaVersion;
+  if (version !== SCHEMA_VERSION) {
+    throw new Error(
+      `This page expects feed v${SCHEMA_VERSION} but the server sent v${String(version)}. Reload to get the current app.`,
+    );
+  }
+
+  const parsed = EventFeed.safeParse(json);
+  if (!parsed.success) {
+    throw new Error("The feed did not match the expected shape.");
+  }
+  return parsed.data;
+}