mobile-connector: merge pairing+devices into one self-service Mobile App page

The two admin-only console pages become a single "Mobile App" page
visible to every logged-in user: connection status (with the last
connection error for troubleshooting), the device list (admin sees all,
others only their own), a pairing dialog with the QR, and — admin-only —
a settings dialog hosting the plugin config, including a relay picker
(official grayed out, test, custom URL). The generic plugin-detail
config form defers to it via the new Plugin::config_in_detail_page flag.

Pairing is now self-service: any user opens a window and the device
auto-binds to them; revocation is admin-for-anyone, owner-for-self;
(re)binding to another user stays admin-only. Binding-managed plugins
(manages_own_access) now expose their non-admin pages to all users and
self-scope per caller (web_pages_for). The relay client records the
error that ends a WS session and clears it on reconnect.

crates/skald-relay-client/src/ws.rs

@@ -46,6 +46,7 @@ pub(crate) async fn run_loop(
             }
             Err(e) => {
                 warn!(crate_name = "skald-relay-client", error = %e, "relay connection ended");
+                state.set_last_error(e.to_string());
             }
         }