Add resolveEl() for shadow DOM compatibility

[jbuckner]

Summary

• Adds BookReader.prototype.resolveEl() that handles el as either a DOM Element or CSS selector string
• Updates mainBRContainer getter in ia-bookreader to use resolveEl()
• Updates search plugin's renderModalMessage to use resolveEl()

Problem

document.querySelector(br.el) fails when:

• br.el is a DOM Element (not a string) — throws SyntaxError: '[object HTMLDivElement]' is not a valid selector
• br.el is a selector like #BookReader but the target element is inside a shadow DOM tree — returns null

This is needed for integrating BookReader into apps that use shadow DOM (e.g. offshoot, which wraps the details page inside app-root's shadow root).

Test plan

• Verify BookReader initializes normally with el: '#BookReader' (string selector, light DOM)
• Verify BookReader initializes with el: document.getElementById('BookReader') (DOM element)
• Verify search modal renders correctly in both modes
• Verify mainBRContainer returns the correct element in both modes

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 28.57143% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.94%. Comparing base (2908b1e) to head (0b63878).

Files with missing lines	Patch %	Lines
src/BookReader.js	25.00%	3 Missing ⚠️
src/plugins/search/view.js	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1520      +/-   ##
==========================================
- Coverage   68.98%   68.94%   -0.05%     
==========================================
  Files          65       65              
  Lines        5556     5561       +5     
  Branches     1229     1233       +4     
==========================================
+ Hits         3833     3834       +1     
- Misses       1689     1693       +4     
  Partials       34       34              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[cdrini]

Can you use this.refs.$br[0] instead of a new method ? That should return the element jquery-wrapped. Only after init is called though; during the setup/constructor BR doesn't really do anything DOM related.

But I do think we can update the types in options.js to allow either a selector or a domelement for el. There are two spots that will need to be updated to switch from .el to this.refs.$br[0].

[jbuckner]

Rebased onto current master (offshoot WEBDEV-8245 shipped today and is now using this patch stack downstream — re-ready for review when someone has a moment 🙏).