feat(web): dashboard All Libraries view saves the global scope

The All Libraries entry in the library switcher could render the
cross-library dashboard but rejected its own settings: the modal
refused to open without a library ('Select a specific library...') and
a save sent library_id: '' (server 400). The server now stores a
global scope (separate table; null/absent library_id, empty string
still 400), so:

- openDashboardSettings allows the All view and fetches prefs without
  the library param (reads the global row; previously a loose NULL
  OR-clause matched an arbitrary library's row).
- saveDashboardSettings sends library_id: null from All Libraries.
- The settings modal gains a scope label (#dashboard-scope-label)
  showing which library — or All Libraries — a save will customize
  (user requirement: always know the scope).

Verified end-to-end in headless Chromium: reorder on All Libraries →
save (null) → reload → modal re-opens with the persisted order and the
All Libraries scope label; per-library save lands in its own row
without touching global (SQL-checked).

web/src/dashboard.ts

@@ -20,14 +20,13 @@ async function openDashboardSettings(): Promise<void> {
   const librarySelect = document.getElementById(
     "library-select",
   ) as HTMLSelectElement;
-  const libraryId = librarySelect?.value;
-  if (!libraryId) {
-    showToast("Select a specific library to customize preferences", "error");
-    return;
-  }
+  const libraryId = librarySelect?.value || "";
 
+  // Empty select = the All Libraries view: its preferences are the GLOBAL
+  // scope — the server returns that row when library_id is absent.
+  const query = libraryId ? `?library_id=${encodeURIComponent(libraryId)}` : "";
   const response = await fetch(
-    `/api/dashboard/preferences?library_id=${libraryId}`,
+    `/api/dashboard/preferences${query}`,
     {
       headers: { Authorization: `Bearer ${localStorage.getItem("token")}` },
     },
@@ -36,6 +35,17 @@ async function openDashboardSettings(): Promise<void> {
   if (response.ok) {
     const prefs = await response.json();
     applyPreferencesToModal(prefs);
+    // Scope indicator: make it unmistakable what a save will customize.
+    const scopeEl = document.getElementById("dashboard-scope-label");
+    if (scopeEl) {
+      const optionText =
+        librarySelect?.selectedOptions[0]?.textContent || libraryId;
+      scopeEl.textContent = (
+        libraryId ? optionText : optionText || "All Libraries"
+      )
+        .replace(/\s*\(\d+\)$/, "")
+        .trim();
+    }
   } else {
     showToast("Failed to load library preferences", "error");
     console.error("API Error:", response.status, response.statusText);
@@ -142,7 +152,10 @@ async function saveDashboardSettings(): Promise<void> {
     ) as HTMLSelectElement;
     const libraryId = librarySelect?.value || "";
     const response = await apiPut("/dashboard/preferences", {
-      library_id: libraryId,
+      // Empty select = the All Libraries view: an explicit JSON null is
+      // the global scope (empty string is rejected by the server as a
+      // lost value — never a scope).
+      library_id: libraryId || null,
       hidden_collections: hiddenCollections,
       collection_order: collectionOrder,
       items_per_section: parseInt(itemsPerCollection),
@@ -150,10 +163,6 @@ async function saveDashboardSettings(): Promise<void> {
     if (response.ok) {
       showToast("Dashboard settings saved", "success");
       closeDashboardSettings();
-      if (!libraryId) {
-        showToast("Select a specific library to customize preferences", "error");
-        return;
-      }
       await fetchAndRenderSections(libraryId);
     }
   } catch (error) {