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

web/src/reader/settings-manager.ts

@@ -78,7 +78,7 @@ export function getDefaultSettings(): ReaderSettings {
     font_size: 18,
     line_height: 1.6,
     margin_width: 20,
-    double_page_spread: false,
+    double_page_spread: true,
     reading_direction: "ltr",
     hardware_acceleration: true,
     panel_layout: {