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).

templates/bookshelf.templ

@@ -133,6 +133,7 @@ templ BookShelf(
 						>
 							<div class="sticky top-0 flex items-center justify-between px-6 py-4 border-b" style="background-color: var(--bg-secondary); border-color: color-mix(in srgb, var(--text-primary) 7%, transparent);">
 								<h2 class="text-lg font-bold" style="color: var(--text-primary)">Filters</h2>
+								<a href="/docs/user/filters" target="_blank" rel="noopener" class="icon-btn" aria-label="Filter guide" title="How filters work">?</a>
 								<button type="button" @click="filtersOpen = false" class="icon-btn" aria-label="Close filters">
 									@Icon("close", "h-5 w-5")
 								</button>