docs(ui): cookie-rendered sidebar state, data-collapsed CSS, working overview links + link-checker gate

Three problems, one root cause each:

1. Sidebar jump: the server rendered its own default (current section
   open, rest collapsed) and restoreDocsNav corrected it after paint —
   open-then-close. Fix: persistence moved server-side. toggleCollapsible
   now writes a docs_nav cookie (strictly-necessary UI state, no
   identifiers, SameSite=Lax); ApplyNavOverrides merges it into the nav
   before render, so first paint IS the visitor's state. The post-paint
   restore is deleted.

2. Arrows out of sync / double-click: two visibility mechanisms
   disagreed. There was NO data-collapsed CSS at all — collapsed sections
   were only ever hidden by JS after paint, and the arrow rotation came
   from a different (server) state. Now: style.css hides on
   [data-collapsed=true] (single source of truth, server-rendered),
   applyCollapsed sets only the attribute, arrows rotate from the same
   merged value. Live check: 37/37 collapsibles arrow-consistent.

3. api-reference.md links: all 20+ quick links were directory-style
   (authentication/ etc.) that 404 on the docs site, plus stale targets
   (books/, ../collections-api.md, websocket 'coming soon'). Rewritten to
   real per-file links — all 9 unique targets return 200 live. Same rot
   fixed in admin/hash-conflicts.md, development.md, android-app.md,
   create_media_item.md, koreader-protocol.md; docs/index.md redirected
   from the deleted collections-api monolith (stale 530-line duplicate of
   the gated api/collections docs — deleted, per the WO2 doctrine).

New gate: TestDocsRelativeLinks walks docs/developer/** and fails make
test on any relative link that is directory-style or unresolvable —
overview links can never silently rot again.

templates/docs.templ

@@ -23,7 +23,7 @@ templ DocsLayout(nav Navigation, doc Document, user User, currentPath string) {
 			<link href="/static/style.css" rel="stylesheet"/>
 			<link rel="icon" type="image/svg+xml" href="/static/favicon.svg"/>
 		</head>
-		<body x-data="docs" x-init="restoreDocsNav(); initializeSearch(); initializeCodeCopyButtons()" class={ "theme-" + user.Theme + " page-docs font-sans antialiased" }>
+		<body x-data="docs" x-init="initializeSearch(); initializeCodeCopyButtons()" class={ "theme-" + user.Theme + " page-docs font-sans antialiased" }>
 			@DocsTopbar(user)
 			<!-- Mobile drawer backdrop -->
 			<div id="docs-overlay" class="hidden fixed inset-0 z-40 lg:hidden" style="background: var(--surface-overlay);" onclick="document.getElementById('docs-sidebar').classList.add('-translate-x-full'); this.classList.add('hidden')"></div>

templates/docs_templ.go

@@ -71,7 +71,7 @@ func DocsLayout(nav Navigation, doc Document, user User, currentPath string) tem
 		if templ_7745c5c3_Err != nil {
 			return templ_7745c5c3_Err
 		}
-		templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 4, "<body x-data=\"docs\" x-init=\"restoreDocsNav(); initializeSearch(); initializeCodeCopyButtons()\" class=\"")
+		templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 4, "<body x-data=\"docs\" x-init=\"initializeSearch(); initializeCodeCopyButtons()\" class=\"")
 		if templ_7745c5c3_Err != nil {
 			return templ_7745c5c3_Err
 		}