Collapse sidebar open/close state to a single per-side cookie derived from the DOM

Problem/Motivation

Sidebar open/close state was kept in three places that had to be manually held in sync: the per-panel {id}_sidebar cookie (server-side rendering), the navigation-plus-hidden DOM class, and an in-memory SidebarManager.active slot reconciled by a tracker behavior. Three stores aligned only by convention is fragile, and it produced occasional, hard-to-pin-down inconsistencies where a panel's shown/hidden state disagreed with what the user expected.

The per-panel cookie model also allowed states the runtime cannot represent: a side has at most one open panel, but each panel had an independent open/closed cookie, so a single side could carry several "open" panels at once.

Proposed resolution

Collapse the three stores to one authoritative source plus its projection:

• Replace the per-panel cookies with one cookie per side (left_sidebar / right_sidebar) holding the id of the open panel, matching the one-panel-per-side invariant.
• Remove the in-memory SidebarManager.active store and the tracker behavior; derive the open panel from the DOM via a new getOpenPanel(side).
• Cache each sidebar panel per user and vary it by exactly the inputs its visibility depends on (cookies:{side}_sidebar, plus navigationMode / activeTool when bound).

API changes

• JS: removed SidebarManager.active; added SidebarManager.getOpenPanel(side). Consumers reading active.{side} must switch to getOpenPanel(side).
• PHP: NavigationPlusUi::isSidebarVisible() now takes (string $id, string $side, ?string $mode, ?string $tool).
• Cookies: per-panel {id}_sidebar replaced by per-side {side}_sidebar. No migration needed; old cookies are ignored.
• Consumers of the sidebar API (e.g. Edit+) need the release carrying this change.