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.

internal/docs/links_test.go

@@ -0,0 +1,90 @@
+package docs
+
+import (
+	"os"
+	"path/filepath"
+	"regexp"
+	"strings"
+	"testing"
+)
+
+// TestDocsRelativeLinks keeps the developer docs tree internally navigable:
+// every relative markdown link must point at a file that exists. The
+// api-reference overview once shipped 20+ directory-style links
+// (authentication/ etc.) that 404'd — this gate makes that class of rot
+// fail make test. (docs/user is excluded: end-user prose has its own pass.)
+//
+// Rules:
+//   - http(s):, mailto:, and pure #anchors are skipped
+//   - every other target must end in .md and resolve (relative to the
+//     linking file) to an existing file inside the tree
+//   - directory-style targets ("area/") are the exact bug this test exists
+//     for and always fail
+var docsLinkRe = regexp.MustCompile(`\[[^\]]*\]\(([^)\s]+)\)`)
+
+func TestDocsRelativeLinks(t *testing.T) {
+	// Walk developer docs; escape-check against the whole docs tree —
+	// cross-area links into docs/user are legitimate (same site).
+	devRoot, err := filepath.Abs("../../docs/developer")
+	if err != nil {
+		t.Fatalf("abs developer root: %v", err)
+	}
+	treeRoot, err := filepath.Abs("../../docs")
+	if err != nil {
+		t.Fatalf("abs docs root: %v", err)
+	}
+	err = filepath.WalkDir(devRoot, func(path string, d os.DirEntry, err error) error {
+		if err != nil || d.IsDir() || !strings.HasSuffix(path, ".md") {
+			return err
+		}
+		raw, err := os.ReadFile(path)
+		if err != nil {
+			return err
+		}
+		rel, _ := filepath.Rel(docsRoot, path)
+		absPath, err := filepath.Abs(path)
+		if err != nil {
+			return err
+		}
+		for _, m := range docsLinkRe.FindAllStringSubmatch(string(raw), -1) {
+			target := m[1]
+
+			if strings.HasPrefix(target, "http://") ||
+				strings.HasPrefix(target, "https://") ||
+				strings.HasPrefix(target, "mailto:") {
+				continue
+			}
+			if strings.HasPrefix(target, "#") {
+				continue // in-page anchor
+			}
+
+			// strip a trailing fragment for file resolution
+			fileTarget := target
+			if i := strings.Index(fileTarget, "#"); i >= 0 {
+				fileTarget = fileTarget[:i]
+			}
+			if fileTarget == "" {
+				continue // pure-anchor form already handled; empty = odd, skip
+			}
+
+			if !strings.HasSuffix(fileTarget, ".md") {
+				t.Errorf("%s: link target %q is not a .md file — directory-style links 404 on the docs site; link the file (e.g. %q)",
+					rel, target, strings.TrimSuffix(fileTarget, "/")+".md")
+				continue
+			}
+
+			resolved := filepath.Clean(filepath.Join(filepath.Dir(absPath), fileTarget))
+			if !strings.HasPrefix(resolved, treeRoot+string(filepath.Separator)) {
+				t.Errorf("%s: link target %q escapes the docs tree", rel, target)
+				continue
+			}
+			if _, err := os.Stat(resolved); err != nil {
+				t.Errorf("%s: link target %q does not resolve to an existing file (%s)", rel, target, resolved)
+			}
+		}
+		return nil
+	})
+	if err != nil {
+		t.Fatalf("walk docs: %v", err)
+	}
+}