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/ui/app.slint

@@ -175,6 +175,7 @@ export component AppWindow inherits Window {
     in property <string> launch-account: "";
     in property <string> launch-root: "";
     in property <string> launch-server: "";
+    in property <string> launch-folder: "";
     in property <bool> launch-busy: false;
     in property <string> launch-status: "";
     in property <string> launch-error: "";
@@ -184,6 +185,8 @@ export component AppWindow inherits Window {
     in-out property <[bool]> launch-format-checked;
 
     callback launch-sign-in(string);
+    /// The path of a folder library — no account, no credential.
+    callback launch-use-folder(string);
     /// server, username, app password
     callback launch-sign-in-direct(string, string, string);
     callback launch-sign-out();
@@ -1228,6 +1231,7 @@ in property <bool> panel-visible: true;
             account: root.launch-account;
             library-root: root.launch-root;
             server-url: root.launch-server;
+            folder-path: root.launch-folder;
             busy: root.launch-busy;
             status: root.launch-status;
             error: root.launch-error;
@@ -1237,6 +1241,7 @@ in property <bool> panel-visible: true;
             format-checked: root.launch-format-checked;
 
             sign-in(server) => { root.launch-sign-in(server); }
+            use-folder(path) => { root.launch-use-folder(path); }
             sign-in-direct(server, user, pw) => {
                 root.launch-sign-in-direct(server, user, pw);
             }