release(v1.7.26-alpha): mirror list + origin-relative download URLs

Adds a multi-mirror manifest fetch. `check_for_updates` walks a
configurable list (data_dir/update-mirrors.json) in priority order
and falls through to the next mirror on any HTTP / parse / timeout
failure. Two defaults bake in: Server 1 (git.tx1138.com) and Server 2
(23.182.128.160:3000).

Critical fix: after parsing a manifest, rewrite every component's
`download_url` so its origin matches the manifest URL we fetched.
Before this, the manifest hard-coded absolute URLs pointing at one
specific server — so even when a node fetched the manifest from a
faster mirror, the actual 200MB download went back to the slow
original. Now the faster mirror wins end-to-end.

New RPCs: update.list-mirrors, update.add-mirror, update.remove-mirror,
update.set-primary-mirror. New UI section on the System Update page
for operator management. 5 new unit tests for origin parsing and
manifest rewriting (21/21 green).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

core/archipelago/src/api/rpc/dispatcher.rs

@@ -431,6 +431,19 @@ impl RpcHandler {
             "update.dismiss" => self.handle_update_dismiss().await,
             "update.download" => self.handle_update_download().await,
             "update.cancel-download" => self.handle_update_cancel_download().await,
+            "update.list-mirrors" => self.handle_update_list_mirrors().await,
+            "update.add-mirror" => {
+                let p = params.unwrap_or(serde_json::json!({}));
+                self.handle_update_add_mirror(&p).await
+            }
+            "update.remove-mirror" => {
+                let p = params.unwrap_or(serde_json::json!({}));
+                self.handle_update_remove_mirror(&p).await
+            }
+            "update.set-primary-mirror" => {
+                let p = params.unwrap_or(serde_json::json!({}));
+                self.handle_update_set_primary_mirror(&p).await
+            }
             "update.apply" => self.handle_update_apply().await,
             "update.git-apply" => self.handle_update_git_apply().await,
             "update.rollback" => self.handle_update_rollback().await,