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.

crates/mtgc-server/Dockerfile

@@ -0,0 +1,24 @@
+# Build from the repository root, so the workspace (and its lockfile) is in
+# the context:
+#
+#   docker build -f crates/mtgc-server/Dockerfile -t mtgc-server .
+
+FROM rust:1-bookworm AS build
+WORKDIR /src
+COPY . .
+RUN cargo build --release --locked -p mtgc-server \
+ && cp target/release/mtgc-server /mtgc-server
+
+FROM debian:bookworm-slim
+RUN apt-get update \
+ && apt-get install -y --no-install-recommends ca-certificates \
+ && rm -rf /var/lib/apt/lists/* \
+ && useradd --system --uid 10001 --home /data mtgc \
+ && mkdir /data && chown mtgc /data
+COPY --from=build /mtgc-server /usr/local/bin/mtgc-server
+USER mtgc
+VOLUME /data
+ENV MTGC_DB=/data/mtgc-server.db \
+    MTGC_BIND=0.0.0.0:8080
+EXPOSE 8080
+CMD ["mtgc-server"]