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/src/lib.rs

@@ -1,4 +1,9 @@
-//! Nextcloud connector — the only [`RemoteBackend`] implementation.
+//! Nextcloud connector.
+//!
+//! One of two [`RemoteBackend`] implementations, registered through
+//! [`NextcloudProvider`]. What an *account* is no longer lives here — that is
+//! [`dr_sync::Account`], which has no server in it — so this crate is the
+//! protocol and nothing else.
 //!
 //! Hand-rolled over `reqwest` rather than built on a WebDAV crate (D7). No
 //! mature Nextcloud crate exists, and the operations that matter here are
@@ -16,11 +21,11 @@ use dr_sync::{
 pub mod auth;
 pub mod desktop_client;
 mod propfind;
-pub mod session;
+pub mod provider;
 
 pub use auth::{AppCredentials, LoginFlow};
 pub use desktop_client::DesktopClient;
-pub use session::{Session, SessionError, SessionStore};
+pub use provider::NextcloudProvider;
 
 /// Chunk sizes Nextcloud's chunked upload v2 accepts.
 const CHUNKS: ChunkConstraints = ChunkConstraints {