WEBDEV-8991: Keep browse loan alive during read aloud

[nsharma123]

Problem

A patron reported their book being auto-returned "due to inactivity" after an hour while actively listening to read aloud — WEBDEV-8991.

ia-book-actions decides a browse loan is still in use from the BookReader:userAction event. That event is triggered in exactly two places today:

• BookReader.prototype.updateFirstIndex() — the displayed index changed
• Navbar.js — a navbar control was clicked

Read aloud produces neither reliably. It only turns a page incidentally, via maybeFlipToIndex(), and in 2up the engine reads both visible pages before it flips — so a single spread can easily outlast the 15 minute window the renewal allows (pageChangedInLast). The renewal check at minute 49 then sees no recorded activity and the loan lapses mid-playback.

This is worse than a late renewal. While the loan is still held, Lending::do_renew takes the $true_renewal branch and skips the availability/policy gate. Once it lapses, the next attempt has to pass that gate — so the book can be lost to another patron outright.

Fix

Drive the activity signal off playback progress rather than the incidental page flip:

• Per chunk — beforeChunkPlay() emits userAction, so every paragraph spoken counts.
• On resume — resuming picks up inside the current chunk, so it would otherwise wait for the next chunk boundary to register.
• Throttled to 60s — chunks are paragraphs; short ones would otherwise signal every few seconds, and each event costs an IndexedDB read+write on the lending side. throttle(fn, threshold, false) fires leading-edge, so the first chunk signals immediately.

No change is needed in ia-book-actions — it already maps userAction onto its page-changed path for renewal purposes.

Testing

• Syntax verified; lint rules reviewed by hand (no max-len; indent/semi/no-unused all satisfied).
• Verified the class-field ordering is safe: BookReaderPlugin's constructor does not call setup() — BookReader.js constructs the plugin, then calls setup() as a separate statement — so signalUserIsActive is initialized before the resume handler is registered.
• ttsEngine.events is a jQuery emitter and AbstractTTSEngine already registers a class-field arrow the same way, so the idiom matches existing code.

I was not able to run the jest suite locally (no node_modules in my checkout); relying on CI here. There is currently no plugin.tts.test.js — happy to add one covering the emit + throttle if reviewers want it in this PR.

Out of scope

The "Are you still there?" warning modal is acknowledge-only — dismissWarningModal() closes it without renewing, and sets a flag that suppresses every later warning. That lives in ia-book-actions and likely deserves its own ticket.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 63.81%. Comparing base (882adbc) to head (559fb68).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1598   +/-   ##
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  Coverage   63.81%   63.81%           
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  Files          69       69           
  Lines        6256     6256           
  Branches     1391     1391           
=======================================
  Hits         3992     3992           
  Misses       2225     2225           
  Partials       39       39           

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