CSP: replace 'unsafe-inline' with a content hash for the theme flash guard

script-src now allow-lists the guard by sha256 of its rendered bytes
(middleware.ScriptHashThemeFlashGuard) instead of 'unsafe-inline' — the
guard is the app's only inline <script>. security_headers_test.go renders
templates.ThemeFlashGuard() and fails with the replacement hash if the
script text ever drifts.

Executing anti-XSS strict CSP surfaced three latent violations the
eval-removal sweep couldn't see (hash mode is stricter than eval mode,
because inline event handler ATTRIBUTES now count against script-src):

1. admin_archived / admin_library: 15 native onclick=/onsubmit= handlers
   (modal open/close, folder-browser picker, purge button) converted to
   Alpine @click/@submit component methods — openCreateLibraryModal,
   hideModal, pickBrowsePath added to the admin component;
   purgeArchivedItems promoted from its window global into the component
   and the (window as any) exposure removed.

2. dashboard/docs: onclick handlers converted — dashboard's
   event.stopPropagation() → @click.stop; docs' sidebar/search closers →
   closeDocsSidebar/closeSearchResults component methods.

3. 8 onerror= image-fallback attributes (cover placeholders app-wide +
   audiobook cover hide) replaced by data-fallback attributes handled by
   one delegated capture-phase listener in web/src/image_fallback.ts —
   works for htmx-swapped content without re-binding.

Verified: theme guard applies a saved theme on public pages under the
hash CSP; 14-page authenticated sweep clean; admin pages clean under the
promoted admin; admin-library modal open/close bindings exercised;
readers render error-free; go build/vet/middleware-test, tsc, vp
check/test (146), build:ts all green.

templates/admin_archived.templ

@@ -36,7 +36,7 @@ templ AdminArchived(user User, items []ArchivedItem, retentionDays int) {
 								</p>
 							</div>
 							if len(items) > 0 {
-								<button type="button" onclick="purgeArchivedItems()" class="btn btn-secondary">
+								<button type="button" @click="purgeArchivedItems()" class="btn btn-secondary">
 									@Icon("trash", "h-4 w-4")
 									Purge All Archived
 								</button>