feat: move the one-click update into the system settings menu

The update console stops being a page of its own and becomes the last section
of 系统设置 (data-section="update", gated by update:read): the sidebar entry
and page-system-update are gone, the console markup lives in
settings-section-update, and the settings menu is where an operator now finds
it. #system-update keeps working as an alias - both hash parsers normalize it
to settings + the update section - so existing deep links and docs keep landing.

Polling follows the new home: leaving the settings page (router) or switching
to another section (settings.js) stops it, selecting the section loads the
console, and returning to the settings page with the section still selected
re-reads the install tree and re-arms the poller (a frozen progress read is
the "nothing is happening" picture an operator must never be shown).

The guard suite follows the move: the old nav/page assertions become "must not
exist", the new ones pin the section, the alias, the poller handoff and the
retired nav.systemUpdate key (settings.nav.update replaces it in zh/EN/RU).
README and both developer/deployment guides say 系统设置 → 一键更新.

docs/en-US/developer-guide.md

@@ -48,7 +48,7 @@ disagree about why an update was refused.
 
 | entry | how it is used |
 |---|---|
-| console | Platform management -> One-click update (`#system-update`). Opening the page reads the local tree only and does not go online; **Check** is what performs the fetch; **Update** starts a job the page polls; the page also carries the "exit after updating" option and a rollback button |
+| console | System settings -> One-click update (`#system-update`, the old deep link still works). Opening this section reads the local tree only and does not go online; **Check** is what performs the fetch; **Update** starts a job the page polls; the page also carries the "exit after updating" option and a rollback button |
 | REST | `GET /api/system/update` (state on disk, no network), `POST /api/system/update/check` (fetch, then report the gap), `POST /api/system/update/apply` (`202` with `job_id`, polled through `GET /api/system/update/job`), `POST /api/system/update/rollback` |
 | CLI | `./cyberstrike-ai -check-update`, `./cyberstrike-ai -update`, `./cyberstrike-ai -update-rollback`. The install root is the directory holding `--config`, or the current directory when `--config` was not given |