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/derived_sync.rs

@@ -1,5 +1,5 @@
 //! TRACES: FR-CAT-3 | FR-CAT-7 | FR-NC-7
-//! Pushing derived state to Nextcloud: thumbnail shards and the catalog.
+//! Pushing derived state to the library: thumbnail shards and the catalog.
 //!
 //! # What travels, and why only this
 //!
@@ -34,8 +34,8 @@
 
 use std::path::{Path, PathBuf};
 
-use dr_sync::{RemoteBackend, RemoteId, RemotePath};
-use dr_sync_nextcloud::AppCredentials;
+use dr_sync::{Connection, RemoteBackend, RemoteId, RemotePath};
+
 use dr_thumbs::ThumbStore;
 
 /// Folder under the library root holding derived state.
@@ -96,8 +96,7 @@ pub enum SyncMessage {
 /// Runs on its own thread with its own runtime, like every other network path
 /// here — the Slint loop must never block (NFR-P9).
 pub fn spawn_sync(
-    creds: AppCredentials,
-    user_id: String,
+    conn: Connection,
     root: String,
     thumbs_dir: PathBuf,
     catalog_path: PathBuf,
@@ -117,7 +116,7 @@ pub fn spawn_sync(
         };
 
         rt.block_on(async {
-            let backend = match crate::remote::connect(&creds, &user_id) {
+            let backend = match crate::remote::connect(&conn) {
                 Ok(b) => b,
                 Err(e) => {
                     let _ = tx.send(SyncMessage::Failed(e.to_string()));