[Belinus-code]

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• My commits are in nice logical chunks with good commit messages
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Description

Fixes #2040.

When the last line of the composition input becomes empty (for example by pressing backspace on the only character of that line, or on the first character of a trailing line after lines were added with ctrl+enter), the line seemed to be deleted and the caret jumped to the end of the previous line.

Cause: in that case Chromium inserts a trailing <br> as a placeholder so the empty last line stays rendered. handleChange() in CompositionInput.tsx removed every <br>. What remained was a text ending in a single \n without a placeholder, which white-space: pre-wrap does not render as an empty line.

Fix: a trailing <br> that follows a text node ending in \n is now replaced by a \n text node instead of being deleted. This is the same placeholder Chromium already inserts when pressing enter at the end of the input (the draft then ends in \n\n, which is why sending trims trailing newlines). Any other <br> is still removed.

Testing:

• Manual: dev build of the app, typed several lines (aaa, newline, b, newline, c) and pressed backspace on the last line. Before the fix the empty line vanished and the caret jumped up (confirmed with debug logging: onInput saw "…\n<br>" and handleChange removed the <br>). After the fix the empty line stays and the caret stays on it.
• No new automated test: the behaviour depends on Chromium's contenteditable DOM and lives inside the component, which the existing unit tests do not cover.
• OS: tested on Ubuntu 24.04 (WSL2 with WSLg), Electron 40 as pinned in this repo. The issue was reported on Windows 10; I could not run a Windows build locally, so a check on Windows would be welcome.