Make storage pluggable, and prove it with a folder backend

`RemoteBackend` existed from the first release and bought nothing it was
designed for. Seven files in `dr-ui` constructed a `NextcloudBackend`
directly, an account *was* a server URL beside a DAV user id, the local
cache directory was named after a hostname, and the launch screen knew
that signing in meant a browser handshake. The trait was real; the seam
was documentation.

A trait over operations is only a quarter of it. Pluggable storage needs
four things, and this adds the other three:

- **Capabilities** — already there, and the reason the engine can drive
  two backends at the speed each actually runs at.
- **Configuration** — `dr_sync::Account`: where a library lives, in
  whatever form its connector addresses, with no server in it. Loads
  every existing config unchanged (`backend` defaults to `nextcloud`,
  `endpoint` is stored under its historical `server` key), and
  `Account::namespace()` reproduces the old catalog directory byte for
  byte, because changing it would abandon a catalog, its thumbnail
  shards, and the sidecars holding unsynced offline work.
- **Registration** — `BackendProvider` and `BackendRegistry`.
  `ui/dr-ui/src/remote.rs` is now the only file above `dr-sync` that
  names a connector.

`Connection` (an account plus an optional `Secret`) replaces the
credentials-and-user-id pair that was threaded through fifteen
signatures in an order that could be swapped. `Secret`'s inner string is
reachable only through `expose()` and its `Debug` prints `Secret(***)`,
so the indirect leak — a `{:?}` on anything holding one — no longer
compiles into a leak.

Nextcloud is unchanged and keeps every peculiarity: propagating ETags,
chunked upload v2, `oc:fileid`, the `oc:permissions` probe on a refused
PUT, the 423 retry classification, Login Flow v2. Those are what the
capability model exists to serve, not something to hide.

`dr-sync-folder` is the second connector: a local disk, a network mount,
an external drive, or a folder a Nextcloud client already syncs. No
account, no credential — the route that works where no secrets daemon
does. It declares `LocalEtags` rather than claiming propagation a POSIX
directory cannot provide, which costs nothing because 50k `stat` calls
are not 50k PROPFINDs. Identity is a path hash, not an inode: an inode
survives a rename but differs between devices and is reused after a
delete, so two machines would disagree about which photograph a
thumbnail belonged to. Re-deriving a thumbnail is a cost; showing the
wrong one is a bug.

docs/storage.md is the contract — the traits, the four steps to add a
backend, and what each connector declares. ARCH §8.0 and §8.4a, and
FR-NC-13, say why.

core/dr-sync-nextcloud/examples/put_probe.rs

@@ -18,7 +18,8 @@
 //! and deleted again, which tests creation and costs nothing.
 
 use dr_plat::PlatformSecretStore;
-use dr_sync_nextcloud::session::SessionStore;
+use dr_sync::AccountStore;
+use dr_sync_nextcloud::NextcloudProvider;
 
 #[tokio::main(flavor = "current_thread")]
 async fn main() {
@@ -30,15 +31,19 @@ async fn main() {
         std::process::exit(2);
     };
 
-    let sessions = SessionStore::open(Box::new(PlatformSecretStore::new()));
+    let sessions = AccountStore::open(Box::new(PlatformSecretStore::new()));
     let Some(session) = sessions
         .current()
-        .filter(|s| s.server == server.trim_end_matches('/'))
+        .filter(|s| s.endpoint == server.trim_end_matches('/'))
     else {
         eprintln!("no stored session for {server}");
         std::process::exit(1);
     };
-    let creds = match sessions.credentials(&session) {
+    let creds = match sessions
+        .connection(&session, true)
+        .map_err(|e| e.to_string())
+        .and_then(|c| NextcloudProvider::credentials(&c).map_err(|e| e.to_string()))
+    {
         Ok(c) => c,
         Err(e) => {
             eprintln!("credentials: {e}");
@@ -48,7 +53,7 @@ async fn main() {
 
     let url = format!(
         "{}/remote.php/dav/files/{}/{}",
-        session.server.trim_end_matches('/'),
+        session.endpoint.trim_end_matches('/'),
         session.user_id,
         path
     );