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/Cargo.lock

@@ -80,7 +80,7 @@ checksum = "a23eb6b1614318a8071c9b2521f36b424b2c83db5eb3a0fead4a6c0809af6e61"
 
 [[package]]
 name = "archipelago"
-version = "1.7.25-alpha"
+version = "1.7.26-alpha"
 dependencies = [
  "anyhow",
  "archipelago-container",