feat(docs): "?" help affordance on every filter mount + docs loader fixes

The filter manual (1ed1602e) was reachable only by knowing /docs
already existed — nothing in the UI pointed at it (the parked manual
plan's open decision). Every filter-builder mount now carries a "?"
in its header linking to the guide (opens in a new tab so drawer and
editor state survive): the All Books drawer and the collection rules
page — the two mounts sharing FilterBuilderRows. The docs home
indexes the manual (guide, cookbook, generated field reference).

Two docs-loader fixes surfaced by pinning this:
- Folder URLs resolve to their index page: /docs/user/filters now
  serves user/filters/index.md (the manual is a folder: guide +
  cookbook + generated reference); previously only root /docs had
  index handling.
- The docs folder path is resolved by walking upward (bounded):
  `go test` runs test binaries from the PACKAGE directory, so the
  relative "docs" path resolved to a folder that didn't exist there
  and every /docs route 404'd in tests while working in prod —
  found by the new pin, fixed so tests exercise the docs routes
  exactly as prod serves them.

Pins: TestFilterHelpAffordance (folder URL serves the guide; drawer
and rules pages carry the link). internal/docs suite + full suite
green (239s).

internal/docs/handler.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"io/fs"
 	"os"
+	"path/filepath"
 	"strings"
 
 	"bookhoard/templates"
@@ -24,7 +25,23 @@ type DocsHandler struct {
 	docsFS   fs.FS
 }
 
+// resolveDocsPath — the docs folder lives at the repo root, but `go test`
+// runs test binaries from the PACKAGE directory (/cmd/server/tests), so a
+// plain relative "docs" resolves to a folder that doesn't exist there and
+// every /docs route 404s in tests (2026-11-01). Walk upward, bounded,
+// until the folder is real; prod (/app) matches on the first try.
+func resolveDocsPath(p string) string {
+	for i := 0; i < 6; i++ {
+		if fi, err := os.Stat(filepath.Join(p, "index.md")); err == nil && !fi.IsDir() {
+			return p
+		}
+		p = filepath.Join("..", p)
+	}
+	return "docs"
+}
+
 func NewDocsHandler(docsPath string) *DocsHandler {
+	docsPath = resolveDocsPath(docsPath)
 	// Create goldmark renderer with syntax highlighting
 	md := goldmark.New(
 		goldmark.WithExtensions(
@@ -74,10 +91,23 @@ func (h *DocsHandler) LoadDocument(docPath string) (*templates.Document, error)
 		}
 	}
 
-	// Read the markdown file
+	// Read the markdown file. Folder URLs resolve to their index page:
+	// /docs/user/filters → user/filters/index.md (2026-11-01 — the
+	// filter manual is a folder: guide + cookbook + generated field
+	// reference).
 	content, err := fs.ReadFile(h.docsFS, docPath)
 	if err != nil {
-		return nil, fmt.Errorf("failed to read document: %w", err)
+		if !strings.HasSuffix(docPath, "index.md") {
+			alt := strings.TrimSuffix(docPath, ".md") + "/index.md"
+			if altContent, altErr := fs.ReadFile(h.docsFS, alt); altErr == nil {
+				docPath = alt
+				content = altContent
+			} else {
+				return nil, fmt.Errorf("failed to read document: %w", err)
+			}
+		} else {
+			return nil, fmt.Errorf("failed to read document: %w", err)
+		}
 	}
 
 	// Convert markdown to HTML