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.

ui/dr-ui/src/identity_ui.rs

@@ -356,15 +356,10 @@ fn to_slint_image(width: u32, height: u32, rgba: &[u8]) -> slint::Image {
 /// boundary would not be.
 /// What the whole-library face pass needs to reach the server.
 ///
-/// Credentials and not just paths, because the pass fetches its own pixels: an
+/// A connection and not just paths, because the pass fetches its own pixels: an
 /// image with no proxy is the ordinary case, not one to skip (see
 /// `library::spawn_face_sweep`).
-pub type SweepPaths = (
-    dr_sync_nextcloud::AppCredentials,
-    String,
-    std::path::PathBuf,
-    std::path::PathBuf,
-);
+pub type SweepPaths = (dr_sync::Connection, std::path::PathBuf, std::path::PathBuf);
 
 /// The detector and embedder files, when both are present.
 pub type ModelPaths = (std::path::PathBuf, std::path::PathBuf);
@@ -690,7 +685,7 @@ pub fn wire<S, M, P>(
             if ctl.regroup.borrow().is_some() {
                 return;
             }
-            let Some((_, _, catalog_path, _)) = paths() else {
+            let Some((_, catalog_path, _)) = paths() else {
                 return;
             };
 
@@ -782,7 +777,7 @@ pub fn wire<S, M, P>(
                 w.set_identity_model_missing(true);
                 return;
             };
-            let Some((creds, user_id, catalog_path, store_dir)) = paths() else {
+            let Some((conn, catalog_path, store_dir)) = paths() else {
                 return;
             };
 
@@ -791,8 +786,7 @@ pub fn wire<S, M, P>(
             *ctl.activity.borrow_mut() =
                 Some(activity.begin(crate::activity::Kind::Index, "Indexing faces"));
             *ctl.sweep.borrow_mut() = Some(crate::library::spawn_face_sweep(
-                creds,
-                user_id,
+                conn,
                 catalog_path,
                 store_dir,
                 detector,