Fix: Ensure mode=2up persists in iframe

[Ankush1oo8]

** Fix for Issue #1311**

This PR ensures that the mode=2up setting is properly respected when the BookReader is embedded inside an iframe. Previously, the reader defaulted to 1up mode despite the URL containing mode=2up.

** Changes Made:**

✅ Updated BookReader.js to ensure that:

• The mode is retrieved from the URL (mode=2up), or from localStorage if previously set.
• Embedded readers (inside iframes) respect the mode=2up parameter.
• Mode switching (1up, 2up, thumb) persists after refresh.
• Navigation controls correctly update localStorage for mode preference.

🛠 Testing Done:

• ✅ Opened BookReader inside an iframe → Confirmed it starts in 2up mode.
• ✅ Changed modes (1up, 2up, thumb) → Refreshed page, mode persisted.
• ✅ Verified fullscreen and non-embedded modes remain unaffected.
• ✅ Tested in Chrome, Firefox, and Edge to confirm cross-browser compatibility.

** Steps to Reproduce & Test:**

1. Embed the reader using:

   <iframe src="https://archive.org/embed/YOUR_ITEM_ID/page/0/mode/2up?ui=embed" width="800" height="600"></iframe>

2. Load in a browser → It should start in 2up mode.
3. Switch to 1up mode → Refresh the page → It should remember the last selected mode.
4. Test outside an iframe (normal mode) → No unintended changes.

✅ Expected Outcome:

• BookReader inside an iframe starts in 2up mode.
• User-selected mode persists after page refresh.
• No impact on normal (non-iframe) usage.

[Ankush1oo8]

can any one check it