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/http_handler.go

@@ -2,8 +2,10 @@ package docs
 
 import (
 	"bytes"
+	"encoding/json"
 	"fmt"
 	"net/http"
+	"net/url"
 	"strings"
 
 	"bookhoard/internal/database"
@@ -46,10 +48,37 @@ func (h *HTTPHandler) currentUser(c *echo.Context) templates.User {
 	return templates.User{Theme: "tokyo-night"}
 }
 
+// docsNavCookie is the browser-saved collapse state of the docs sidebar
+// (set by web/src/docs.ts on every toggle). Strictly-necessary UI state:
+// first-party, no identifiers, lets the server render the sidebar in the
+// visitor's chosen state so there is no post-paint jump.
+const docsNavCookie = "docs_nav"
+
+// docsNavOverrides reads the saved collapse state from the cookie.
+// Keys are the sidebar's data-key values ("Section", "Section / Group");
+// values are the collapsed flag. Malformed cookies are ignored — the
+// server-rendered defaults win.
+func docsNavOverrides(c *echo.Context) map[string]bool {
+	cookie, err := c.Cookie(docsNavCookie)
+	if err != nil || cookie.Value == "" {
+		return nil
+	}
+	raw, err := url.QueryUnescape(cookie.Value)
+	if err != nil {
+		return nil
+	}
+	var overrides map[string]bool
+	if err := json.Unmarshal([]byte(raw), &overrides); err != nil {
+		return nil
+	}
+	return overrides
+}
+
 // renderDocPage renders a document inside the docs layout.
 func (h *HTTPHandler) renderDocPage(c *echo.Context, doc *templates.Document) error {
 	currentPath := c.Request().URL.Path
 	nav := h.docs.BuildNavigation(currentPath)
+	ApplyNavOverrides(nav, docsNavOverrides(c))
 
 	var buf bytes.Buffer
 	if err := templates.DocsLayout(*nav, *doc, h.currentUser(c), currentPath).Render(c.Request().Context(), &buf); err != nil {