feat(library-switcher): add shared library switcher component and module

Add reusable library switcher infrastructure that can be used across
dashboard, collections list, and collection detail pages.

New files:
- templates/library_switcher.templ: Shared LibrarySwitcher component
  with variadic actions slot for page-specific buttons (e.g. dashboard
  settings/refresh). Includes DashboardActions sub-component.
- web/src/library-switcher.ts: Shared module providing:
  - initLibrarySwitcher(): syncs dropdown with localStorage, attaches
    change listener with configurable onSwitch callback
  - switchWithTransition(): generic fade-out -> spinner -> fetch ->
    fade-in transition used by all pages
  - getCurrentLibraryId(): reads "selectedLibrary" from localStorage

Modified:
- web/src/main.ts: import new library-switcher module
- web/src/types/api.d.ts: add book_count field to CollectionData

templates/library_switcher.templ

@@ -0,0 +1,59 @@
+package templates
+
+templ LibrarySwitcher(libData []LibraryData, currentLibraryID string, actions ...templ.Component) {
+	<div class="sticky top-0 z-40 bg-opacity-95 backdrop-blur border-b" style="background-color: var(--bg-primary);">
+		<div class="w-full px-4 py-3 flex items-center justify-between">
+			<div class="flex items-center gap-4">
+				<label class="text-sm font-medium" style="color: var(--text-secondary)">Library:</label>
+				<select
+					id="library-select"
+					name="library_id"
+					class="px-4 py-2 rounded-lg border focus:ring-2 focus:ring-blue-500"
+					style="background-color: var(--bg-secondary); color: var(--text-primary);"
+				>
+					<option value="">All Libraries</option>
+					for _, lib := range libData {
+						if lib.ID == currentLibraryID {
+							<option value={ lib.ID } selected>{ lib.Name }</option>
+						} else {
+							<option value={ lib.ID }>{ lib.Name }</option>
+						}
+					}
+				</select>
+			</div>
+			if len(actions) > 0 {
+				<div class="flex items-center gap-2">
+					for _, action := range actions {
+						@action
+					}
+				</div>
+			}
+		</div>
+		<div
+			id="loading-spinner"
+			class="hidden fixed inset-0 bg-opacity-50 flex items-center justify-center z-50"
+			style="background-color: var(--bg-primary);"
+		>
+			<div class="animate-spin rounded-full h-12 w-12 border-b-2" style="border-color: var(--accent);"></div>
+		</div>
+	</div>
+}
+
+templ DashboardActions() {
+	<button
+		data-action="open-dashboard-settings"
+		class="p-2 rounded-lg hover:bg-gray-700 transition-colors"
+		style="background-color: var(--bg-secondary);"
+		title="Customize Dashboard"
+	>
+		⚙️
+	</button>
+	<button
+		data-action="reload-page"
+		class="p-2 rounded-lg hover:bg-gray-700 transition-colors"
+		style="background-color: var(--bg-secondary);"
+		title="Refresh"
+	>
+		🔄
+	</button>
+}