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

internal/services/reader.go

@@ -403,6 +403,7 @@ func (s *ReaderService) getDefaultSettings() map[string]interface{} {
 		"tap_zone_size":      30,
 		"auto_scroll":        false,
 		"panel_zoom_enabled": true,
+		"double_page_spread": true,
 
 		// Dockable panel defaults
 		"panel_layout": map[string]interface{}{