feat: Add share-link backend and wire up the sheet

Add a small Go + SQLite service (server/) that stores character-sheet
JSON blobs and returns short, content-addressed IDs, so sheets can be
shared via a compact ?s=<id> link instead of an oversized inline payload.

CharacterSheet.tsx now POSTs to the service on share and fetches by ID on
load, falling back to the self-contained ?c= inline link when the backend
is unreachable. Legacy ?c= links still decode.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/CharacterSheet.tsx

@@ -185,6 +185,13 @@ function pruneEmpty(data: Record<string, unknown>) {
   );
 }
 
+// Resolve a share-service path relative to the current page so it works under
+// any base path (e.g. /fabula/) where Caddy reverse-proxies /api/* to the
+// backend. Returns e.g. https://host/fabula/api/s.
+function apiURL(path: string) {
+  return new URL(path, window.location.href).toString();
+}
+
 export default function CharacterSheet() {
   const [activeTab, setActiveTab] = useState("main");
   const [urlMode, setUrlMode] = useState(false);
@@ -397,8 +404,23 @@ export default function CharacterSheet() {
 
   // Init
   useEffect(() => {
-    const encoded = new URLSearchParams(window.location.search).get("c");
-    if (encoded) {
+    const params = new URLSearchParams(window.location.search);
+    const shortId = params.get("s");
+    const encoded = params.get("c");
+    if (shortId) {
+      // Short link: fetch the sheet JSON from the share service.
+      fetch(apiURL("api/s/" + encodeURIComponent(shortId)))
+        .then((res) => {
+          if (!res.ok) throw new Error(String(res.status));
+          return res.json();
+        })
+        .then((data) => {
+          applyData(data);
+          setUrlMode(true);
+        })
+        .catch(() => tryAutoLoad());
+    } else if (encoded) {
+      // Legacy/offline self-contained link with the data inline.
       decompressFromBase64(encoded)
         .then((json) => {
           applyData(JSON.parse(json));
@@ -477,12 +499,24 @@ export default function CharacterSheet() {
   );
 
   const copyShareURL = useCallback(async () => {
-    const encoded = await compressToBase64(
-      JSON.stringify(pruneEmpty(collectData())),
-    );
-    await navigator.clipboard.writeText(
-      window.location.origin + window.location.pathname + "?c=" + encoded,
-    );
+    const json = JSON.stringify(pruneEmpty(collectData()));
+    const base = window.location.origin + window.location.pathname;
+    let shareURL: string;
+    try {
+      // Preferred: store the sheet server-side and share a short ?s=<id> link.
+      const res = await fetch(apiURL("api/s"), {
+        method: "POST",
+        headers: { "Content-Type": "application/json" },
+        body: json,
+      });
+      if (!res.ok) throw new Error(String(res.status));
+      const { id } = await res.json();
+      shareURL = base + "?s=" + id;
+    } catch {
+      // Backend unreachable: fall back to a self-contained inline ?c= link.
+      shareURL = base + "?c=" + (await compressToBase64(json));
+    }
+    await navigator.clipboard.writeText(shareURL);
     setCopyStatus(true);
     setTimeout(() => setCopyStatus(false), 2000);
   }, [collectData]);