feat(docs): standalone docs site chrome + category/subcategory navigation

Docs pages no longer embed the app Header (whose hx-post login form silently
degraded to a native GET, leaking credentials into the URL — this page never
loaded htmx). New docs-owned topbar: search, theme picker, Open App; anonymous
visitors get a plain /login link. Optional-auth middleware (hand-rolled —
echojwt's ErrorHandler return ends the chain, producing empty 200s) populates
the user for signed-in visitors.

Navigation rebuilt from the docs directory tree: 6 sections, subcategory
groups, server-rendered expansion and active states. Search results are a
templ partial (/docs/search/partial); code highlighting is server-side chroma
(previously dead hljs CSS); prose is theme-aware via typography DEFAULT vars
(prose-invert painted unreadable text under light themes). /docs renders the
index directly; standalone 404 without the tailwind CDN.

Tests: navigation routing/expansion (internal/docs), render pins asserting no
login form/password field/app sidebar ever return on docs pages.
lint-rendering: exempt *.test.ts fixtures (jsdom DOM building); docs.ts
baseline 5 -> 1.

templates/utils.go

@@ -6,9 +6,9 @@ import (
 	"bookhoard/internal/utils"
 	"encoding/json"
 	"fmt"
-	"strconv"
 	"net/url"
 	"sort"
+	"strconv"
 	"strings"
 	"time"
 
@@ -33,6 +33,35 @@ func navItemClass(current, target string) string {
 	return base + "text-content-muted hover:text-content hover:bg-surface-hover"
 }
 
+// docsNavClass returns classes for docs sidebar links (server-rendered active
+// state — there is no client-side highlighter). indent sets the left padding,
+// deeper for subcategory group items.
+func docsNavClass(current, target, indent string) string {
+	base := "nav-item flex items-center " + indent + " pr-4 mr-2 py-1.5 rounded-lg text-sm transition-colors "
+	if strings.TrimRight(current, "/") == strings.TrimRight(target, "/") {
+		return base + "bg-brand/15 text-brand font-medium"
+	}
+	return base + "text-content-muted hover:text-content hover:bg-surface-hover"
+}
+
+// collapsibleState renders the data-collapsed attribute value for a docs nav
+// items container; web/src/docs.ts toggles it on click.
+func collapsibleState(collapsed bool) string {
+	if collapsed {
+		return "true"
+	}
+	return "false"
+}
+
+// chevronStyle pre-rotates a docs nav section/group chevron when collapsed
+// (quote-free value: templ double-escapes quotes in style attributes).
+func chevronStyle(collapsed bool) string {
+	if collapsed {
+		return "transform: rotate(-90deg);"
+	}
+	return "transform: rotate(0deg);"
+}
+
 // themeCheckClass returns the class suffix for a theme option's checkmark
 // wrapper: empty for the active theme (visible), " hidden" otherwise. The
 // check element is always rendered so updateThemeIndicators() can move it