docs(ui): drop sidebar saved-state — server defaults already ARE the standard docs behavior

Simplification per spec: the menu tracks state through the URL alone.
Land on /docs: fully closed, headers only (an explicit home rule now
overrides the contains-URL rule, since the root overview doc lives in
Getting Started and would otherwise auto-open it). Land on any doc page:
its containing section + group render open at first paint, current link
bold — no JS, no jump, arrows consistent. Navigate elsewhere: old
closes, new opens.

Deleted: docs_nav cookie const/parse, ApplyNavOverrides, cookie write +
collect/persist helpers in web/src/docs.ts, data-key attributes in the
sidebar templ, and the first-section-open fallback (docs home used to
force-open the first section — spec'd as closed).

toggleCollapsible remains for in-page peeking; purely ephemeral.
The [data-collapsed=true] CSS from the previous round stays — it is what
makes the server-rendered state actually hide sections at paint (the
original bug was that this CSS never existed).

Tests: TestBuildNavigationNothingCurrent now asserts zero sections open
on unmatched paths; TestBuildNavigation asserts Getting Started closed
on a device page (no site-wide pin). make test green; 146 vitests; live
verified: home 0 expanded, doc pages exactly 2 (section+group).

templates/docs.templ

@@ -198,7 +198,7 @@ templ DocsSidebar(nav Navigation, currentPath string) {
 							@Icon("chevron-down", "h-4 w-4")
 						</span>
 					</button>
-					<div class="nav-items pb-1" data-key={ section.Title } data-collapsed={ collapsibleState(section.Collapsed) }>
+					<div class="nav-items pb-1" data-collapsed={ collapsibleState(section.Collapsed) }>
 						for _, item := range section.Items {
 							<a href={ item.URL } class={ docsNavClass(currentPath, item.URL, "pl-7") }>
 								<span>{ item.Title }</span>
@@ -217,7 +217,7 @@ templ DocsSidebar(nav Navigation, currentPath string) {
 										@Icon("chevron-down", "h-3.5 w-3.5")
 									</span>
 								</button>
-								<div class="nav-group-items" data-key={ section.Title + " / " + group.Title } data-collapsed={ collapsibleState(group.Collapsed) }>
+								<div class="nav-group-items" data-collapsed={ collapsibleState(group.Collapsed) }>
 									for _, item := range group.Items {
 										<a href={ item.URL } class={ docsNavClass(currentPath, item.URL, "pl-9") }>
 											<span class="truncate">{ item.Title }</span>