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).

templates/dashboard.templ

@@ -172,6 +172,7 @@ templ DashboardSettingsModal(sections []handlers.SectionData, hiddenCollections
 				<div>
 					<h2 class="text-xl font-bold" style="color: var(--text-primary)">Customize Dashboard</h2>
 					<p class="text-sm mt-1" style="color: var(--text-secondary)">Drag to reorder, toggle to show or hide.</p>
+					<p class="text-xs mt-1 font-medium" id="dashboard-scope-label" style="color: var(--accent)"></p>
 				</div>
 				<button data-action="close-dashboard-settings" class="icon-btn" aria-label="Close">
 					@Icon("close", "h-5 w-5")