Replace folder sync with an HTTP sync server, keeping the app offline-first

Folder sync left too much to the user's sync tool: platform file access
(the Android SAF plugin), choosing the document, merging conflict copies —
and its merge base could fall out of step with the document (absorbing
conflict copies, "forget", picking a new Android folder), reading every
difference as a deletion.

The new mtgc-server crate stores one versioned library per user and accepts
a write only when it names the version it replaces. It never merges: devices
still do that with merge3, through a new sync cycle over a `Remote` trait in
mtgc-core. The base advances only on knowledge — to what was read, or to a
write the server confirmed — and is stored with the server and account it
belongs to, so a different account starts from an empty ancestor instead of
deleting. The working copy is only rewritten under the app-state lock, and
sync commands now run off the main thread.

Sign-in happens in the browser on the server's pages, via the owner's OIDC
provider and/or local username/password accounts (Argon2id, managed with
`mtgc-server user ...`), and returns a per-device revocable token to the app
by PKCE-protected code: a loopback port on desktop, the mtgc:// scheme on
Android.

The Info tab gains sign-in, the first-sync combine/replace choice, the
account's devices, and JSON export/import; the top bar shows sync state.
Includes a Dockerfile, Compose/Caddy example, and scripts to build and push
${DOCKER_REGISTRY}/mtgc/sync-server.

src/components.tsx

@@ -1767,3 +1767,18 @@ function IconToggle({
     </button>
   );
 }
+
+/** Compact relative time, e.g. "2h ago" — for "updated"/"synced" stamps. */
+export function formatRelativeTime(epochSeconds: number): string {
+  const diffSec = Math.round((Date.now() - epochSeconds * 1000) / 1000);
+  if (diffSec < 30) return "just now";
+  const mins = Math.round(diffSec / 60);
+  if (mins < 60) return `${mins}m ago`;
+  const hours = Math.round(mins / 60);
+  if (hours < 24) return `${hours}h ago`;
+  const days = Math.round(hours / 24);
+  if (days < 30) return `${days}d ago`;
+  const months = Math.round(days / 30);
+  if (months < 12) return `${months}mo ago`;
+  return `${Math.round(months / 12)}y ago`;
+}