Reflect parent-side line-text edits in real time

Surfaced during testing of the `5-2-tools-align` cut (this repo's PR #6, authority PR CenterForDigitalHumanities/TPEN-interfaces#564). Preview-Transcription only shows the line text it fetches on boot from `annotationPage`, plus values it sends itself via `UPDATE_LINE_TEXT`. Edits made directly in the parent transcription-block do not propagate, so the preview drifts stale until the tool is reloaded.

This is a pre-existing gap, not introduced by PR #6.

## Required change

Pending the parent-side broadcaster (CenterForDigitalHumanities/TPEN-interfaces — see companion issue), add a case for `LINE_TEXT_UPDATED { lineId, text }` in the `window` `message` listener in `index.html`. Locate the row by `lineId` and update its `` value. The existing `highlightLine(lineId)` lookup pattern is the right shape to re-use.

## Companion issue

CenterForDigitalHumanities/TPEN-interfaces#565

Start in index.html at the window message listener and review the existing highlightLine(lineId) lookup pattern. Add handling for LINE_TEXT_UPDATED { lineId, text } so the matching input reflects the new text. Done means parent-side line edits update the preview without reloading, once the companion broadcaster is available.

Written by the indexing model from the issue text.