feat(reader): wire double-page spread setting into web reader

The double_page_spread checkbox in the reader settings panel was inert:
it had no Alpine binding, no apply logic, and no persistence. Default
was also inconsistent (false in settings-manager, absent from server
defaults).

- Add doublePageSpread state to the reader Alpine component, loaded
  from saved settings (default true)
- Add applyDoublePageSpread() which sets the renderer's 'spread'
  attribute to auto/none and persists the setting via saveSettings
- Apply the spread attribute during fixed-layout renderer init
- Bind the settings checkbox with x-model and @change
- Add double_page_spread: true to ReaderService server defaults so
  new users get the same starting value the client expects
- Also improve the PDF pan/select toolbar button: distinct smart-
  select vs pan icons, highlighted state while pan mode is active,
  and dynamic tooltips/aria-labels explaining each mode

templates/reader.templ

@@ -153,8 +153,13 @@ templ ReaderChrome(user User, metadata ReaderMetadata, progress ReadingProgress)
 							</svg>
 						</button>
 						<!-- Pan/Select mode (PDF only) -->
-						<button x-show="isPDF" @click="toggleInteractionMode()" class="p-1.5 sm:p-2 rounded-lg hover:bg-gray-700" title="Pan/Select Mode" aria-label="Toggle pan/select mode">
-							<svg class="reader-icon" width="20" height="20" aria-hidden="true">
+						<button x-show="isPDF" @click="toggleInteractionMode()" class="p-1.5 sm:p-2 rounded-lg hover:bg-gray-700" :class="interactionMode === 'pan' ? 'bg-blue-600 hover:bg-blue-700' : ''" :title="interactionMode === 'pan' ? 'Pan mode: drag anywhere to move. Click for Smart select.' : 'Smart select: drag text to select, drag blank to pan (Shift = force pan). Click for Pan mode.'" :aria-label="interactionMode === 'pan' ? 'Pan mode' : 'Smart select mode'">
+							<!-- Smart select (default): text cursor -->
+							<svg x-show="interactionMode !== 'pan'" class="reader-icon" width="20" height="20" aria-hidden="true">
+								<path d="M 10 3 L 10 17 M 7 5 L 10 4 L 13 5 M 7 15 L 10 16 L 13 15"></path>
+							</svg>
+							<!-- Force pan: move arrow -->
+							<svg x-show="interactionMode === 'pan'" class="reader-icon" width="20" height="20" aria-hidden="true">
 								<path d="M 5 3 v 12 M 5 15 l -2 2 M 5 15 l 2 2 M 5 3 l 3 3"></path>
 							</svg>
 						</button>
@@ -306,7 +311,7 @@ templ ReaderSettingsPanel() {
 			<div class="mb-6" x-show="isFixedLayout">
 				<h3 class="font-semibold mb-2">Navigation</h3>
 				<label class="flex items-center mb-2">
-					<input type="checkbox" name="double_page_spread" class="mr-2"/>
+					<input type="checkbox" name="double_page_spread" x-model="doublePageSpread" @change="applyDoublePageSpread()" class="mr-2"/>
 					Double Page Spread
 				</label>
 			</div>