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).

internal/docs/navigation_test.go

@@ -127,8 +127,8 @@ func TestBuildNavigation(t *testing.T) {
 	}
 
 	getting := nav.Sections[0]
-	if getting.Collapsed {
-		t.Error("Getting Started should be expanded (root overview pinned)")
+	if !getting.Collapsed {
+		t.Error("Getting Started should be collapsed: only sections containing the current page open")
 	}
 	if len(getting.Items) != 1 || getting.Items[0].Title != "Overview" || getting.Items[0].URL != "/docs" {
 		t.Errorf("Getting Started items = %+v, want single Overview → /docs", getting.Items)
@@ -196,10 +196,7 @@ func TestBuildNavigationNothingCurrent(t *testing.T) {
 			open++
 		}
 	}
-	if open != 1 {
-		t.Errorf("expected exactly one section open on a 404 path, got %d", open)
-	}
-	if nav.Sections[0].Collapsed {
-		t.Error("fallback should open the first section")
+	if open != 0 {
+		t.Errorf("expected zero sections open when nothing matches (docs home / 404 renders fully closed), got %d", open)
 	}
 }