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.

web/src/admin.ts

@@ -427,8 +427,6 @@ async function purgeArchivedItems(): Promise<void> {
   }
 }
 
-(window as any).purgeArchivedItems = purgeArchivedItems;
-
 // Restore a hidden/archived item to library visibility without waiting for
 // its file to return (POST /api/media-items/:id/unarchive). If the file is
 // still gone the next scan hides it again.
@@ -599,6 +597,21 @@ Alpine.data("admin", () => ({
     this.loadWatchStatus();
     this.checkActiveScans();
   },
+  purgeArchivedItems,
+  openCreateLibraryModal() {
+    document.getElementById("create-library-modal")?.classList.remove("hidden");
+  },
+  hideModal(id: string) {
+    document.getElementById(id)?.classList.add("hidden");
+  },
+  pickBrowsePath(e: Event) {
+    const button = e.currentTarget as HTMLElement;
+    const input = document.getElementById(
+      button.dataset.targetInput ?? "",
+    ) as HTMLInputElement | null;
+    if (input) input.value = button.dataset.currentPath ?? "";
+    this.hideModal("folder-browser-modal");
+  },
   cancelAllScans,
   checkActiveScans,
   hideScanProgress,