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/CommandPalette.tsx

@@ -3,7 +3,7 @@ import { api, CardRef, Deck } from "./api";
 import { DialogFrame, useClickPreview } from "./components";
 
 // Kept in step with App.tsx's Tab union by hand — it isn't exported.
-type Tab = "add" | "library" | "search" | "decks" | "news";
+type Tab = "add" | "library" | "search" | "decks" | "news" | "info";
 
 interface PaletteItem {
   key: string;
@@ -38,15 +38,13 @@ function subsequenceMatch(query: string, text: string): boolean {
 export function CommandPalette({
   onNavigate,
   onUpdateCardData,
-  onOpenLibrary,
-  onNewLibrary,
+  onSyncNow,
   undo,
   onRunUndo,
 }: {
   onNavigate: (tab: Tab) => void;
   onUpdateCardData: () => void;
-  onOpenLibrary: () => void;
-  onNewLibrary: () => void;
+  onSyncNow: () => void;
   /** The most recent undoable action, if its window hasn't lapsed yet. */
   undo: { message: string; run: () => void | Promise<void> } | null;
   onRunUndo: () => void;
@@ -98,8 +96,8 @@ export function CommandPalette({
     { key: "nav-decks", label: "Go to Decks", run: () => onNavigate("decks") },
     { key: "nav-news", label: "Go to News", run: () => onNavigate("news") },
     { key: "update-card-data", label: "Update card data", run: onUpdateCardData },
-    { key: "open-library", label: "Switch library file…", run: onOpenLibrary },
-    { key: "new-library", label: "Create new library file…", run: onNewLibrary },
+    { key: "sync-now", label: "Sync library now", run: onSyncNow },
+    { key: "nav-sync", label: "Sync settings", run: () => onNavigate("info") },
   ];
 
   const items: PaletteItem[] = [