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/reader.ts

@@ -371,6 +371,7 @@ document.addEventListener("alpine:init", () => {
     isPDF: false,
     interactionMode: "select" as string,
     magnifierEnabled: false,
+    doublePageSpread: true as boolean,
     progressText: "",
     progressLabel: "",
     progressMain: "",
@@ -454,6 +455,7 @@ document.addEventListener("alpine:init", () => {
         this.readingFont = this.settings.reading_font || "literata";
         this.fontSize = this.settings.font_size || 18;
         this.lineHeight = this.settings.line_height || 1.6;
+        this.doublePageSpread = this.settings.double_page_spread ?? true;
         if (this.settings.reading_mode) {
           this.readingMode = this.settings.reading_mode;
         } else {
@@ -492,6 +494,7 @@ document.addEventListener("alpine:init", () => {
           this.zoomPercent = this.renderer.zoomPercent;
         });
         this.computeFixedLayoutChapterBoundaries();
+        this.applyDoublePageSpread();
       } else {
         this.renderer.setStyles?.(this.buildCSS());
       }
@@ -692,6 +695,14 @@ document.addEventListener("alpine:init", () => {
       this.renderer.setAttribute("interaction-mode", next);
       this.interactionMode = next;
     },
+    applyDoublePageSpread() {
+      if (!this.isFixedLayout || !this.renderer) return;
+      this.renderer.setAttribute(
+        "spread",
+        this.doublePageSpread ? "auto" : "none",
+      );
+      saveSettings({ double_page_spread: this.doublePageSpread });
+    },
     goLeft() {
       this.view?.goLeft?.();
     },

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: {