Add magnifier, interaction mode toggle, and zoom percentage display

Magnifier (fixed-layout.js):
- Rewrite magnifier to use CSS background-image for comics (blob URLs
  from iframe img elements) and canvas drawImage for PDFs. Replaces
  broken iframe.cloneNode approach that created empty sandboxed clones.
- Detect which iframe is under the cursor by checking bounding rects,
  so magnifier works on both left and right panes.
- Forward mousemove events from iframe documents to handleMagnifierMove
  so the magnifier follows the cursor inside iframes.
- Add background-repeat: no-repeat to lens styles.
- Add zoomMagnifierEnabled getter for external state checks.

Interaction mode toggle (PDF only):
- Add 'interaction-mode' observed attribute with 'select'/'pan' values.
- In select mode (default for PDFs): mouse events pass through for text
  selection; Shift+drag enables panning as a fallback.
- In pan mode: drag-to-pan works as normal.
- Comic books always use pan mode (no text to select).
- Auto-detect PDF via #isPDF flag set in #showSpread from onZoom presence.
- Add isPDF getter so reader.js can show/hide the toggle button.
- Add toggle button in reader.html, hidden by default, shown for PDFs
  via the view 'load' event.
- Escape key dismisses the magnifier.

Zoom percentage display (reader.js):
- Dispatch 'zoom' CustomEvent from #zoomByRatio with the new scale.
- Listener in reader.js updates the percentage display on scroll zoom.

Also includes minor formatting (line wrapping) in #goLeft/#goRight.

reader.html

@@ -365,6 +365,16 @@
         <path d="M 14 14 L 18 18" />
       </svg>
     </button>
+    <button
+      id="interaction-mode-toggle"
+      aria-label="Toggle pan/select mode"
+      title="Pan Mode"
+      style="display: none"
+    >
+      <svg class="icon" width="20" height="20" aria-hidden="true">
+        <path d="M 5 9 h 10 M 10 5 v 10" />
+      </svg>
+    </button>
   </div>
 </div>
 <script src="reader.js" type="module"></script>