Fix background color removal failing on second toggle when text formatting is present

[Copilot]

After applying background color + any text formatting (underline, bold, etc.), removing the background works once but silently fails on subsequent remove attempts.

Root Cause

The cleanup dispatch in formatSelection.ts reads from Redux state to identify background-color artifacts to strip. However, the state is stale at cleanup time — the Editable component throttles its state sync by 500ms, so the state still holds the previous custom color (e.g. red) rather than the colors.bg value that execCommand just wrote to the DOM.

On the first removal this is benign: the DOM visually looks correct after the throttled sync. But on re-apply, Chromium creates a nested structure because the dirty background-color: colors.bg artifact left in state causes execCommand to add the new color as an inner element rather than replacing the outer one:

<!-- State after first removal (dirty): -->
<font color="#000000" style="background-color: rgb(0,0,0);">
  <u><font style="background-color: rgba(255,87,61,1);">Hello</font></u>
</font>

<!-- After second removal attempt, cleanup removes the outer colors.bg but not the inner red: -->
<font color="#000000">
  <u><font style="background-color: rgba(255,87,61,1);">Hello</font></u>
</font>
<!-- ↑ Red background still visible -->

Fix

Read contentEditable.innerHTML (immediately reflects execCommand output) instead of the stale Redux state value when computing what to strip. The state value is still used as oldValue for editThought to keep lexeme updates correct.

// Before: reads stale Redux state — misses the inner custom color
const styleRemovedThought = value.replace(styleAttrPattern, ...)

// After: reads current DOM — all background-color: colors.bg artifacts are present and stripped
const currentHtml = contentEditable?.innerHTML ?? value
const styleRemovedThought = currentHtml.replace(styleAttrPattern, ...)

Changes

• src/actions/formatSelection.ts: Use contentEditable?.innerHTML as the source for cleanup computation in the backColor/bg dispatch
• src/e2e/puppeteer/__tests__/color.ts: Add test covering background + text formatting + remove + re-apply + remove cycle

[raineorshine]

Wait for #4048