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/http_handler.go

@@ -2,10 +2,8 @@ package docs
 
 import (
 	"bytes"
-	"encoding/json"
 	"fmt"
 	"net/http"
-	"net/url"
 	"strings"
 
 	"bookhoard/internal/database"
@@ -48,37 +46,10 @@ 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 {