Upgrade accounts saved as http:// to https on launch

Before the previous commit, browser sign-in could store an account as
http://, and the client now refuses to send to one. Left alone, such a
library would fail to open with a configuration error, so its stored
endpoint is rewritten before anything reads it.

The endpoint is half of two keys, and both are handled:

- The keyring entry is filed under it. Rewriting only the record would
  strand the app password under the old key and sign the user out, so
  AccountStore::move_endpoint copies the secret across first, rewrites
  the record in place (the last record is the one resumed), and deletes
  the old entry only once nothing refers to it.
- namespace() is built from it and names the catalog directory. For an
  http to https rewrite it does not change, because the namespace strips
  either scheme. A move that would change it is refused, not performed,
  so no later rewrite can abandon a catalog either.

The rewrite is a new BackendProvider::upgrade_endpoint hook, which does
nothing by default, and not a second call to normalise_endpoint. The
folder connector's normalise_endpoint canonicalises the path and needs
it to exist, so running it on every launch would fail a library on an
unplugged disk, or rename one whose path now resolves differently. Only
Nextcloud implements the hook.

If the move fails (for example, a locked keyring), it is logged, the
account is left as it was, and the move is tried again on the next
launch.

Closes #65.

ui/dr-ui/src/launch_ui.rs

@@ -29,6 +29,10 @@ pub struct LaunchController {
 impl LaunchController {
     pub fn new() -> Rc<Self> {
         let store = AccountStore::open(Box::new(PlatformSecretStore::new()));
+        // Before anything reads an account: an endpoint an older build stored
+        // as `http://` is refused by the client, so resuming it unrewritten
+        // would fail the library it names (NFR-SEC-3).
+        store.upgrade_endpoints(crate::remote::registry());
         let model = LaunchModel::from_store(&store);
         Rc::new(Self {
             model: RefCell::new(model),