[TASK] DPL-177: Make readability bar a lit component

[sbuerk]

What

Encapsulates the CKEditor "DeepL Write" readability progress bar as a
dedicated lit web component (<deepl-write-readability>), removing the
inline <style> block and the style="--value" attributes from the Core13
and Core14 CkEditor/Edit.html templates.

Why

Inline <style> elements and style="" attributes are blocked by a tightened
TYPO3 backend Content-Security-Policy (style-src without 'unsafe-inline')
and are poor practice regardless.

How

• New plain-JS lit component
  Resources/Public/JavaScript/Ckeditor/readability-progress.js, registered in
  the importmap (Configuration/JavaScriptModules.php).
• The CSS lives in the Shadow DOM (full isolation from other components) and is
  emitted as a nonce-carrying <style> via the core styleTag() helper
  (@typo3/core/lit-helper.js), so it is CSP compliant (style-src 'nonce-...').
• A constructable stylesheet (static styles) cannot be used here:
  @typo3/backend/modal.js relocates the overlay content into the top
  backend document, so a stylesheet constructed in the FormEngine iframe
  realm cannot be adopted into the top-document shadow root
  (Adopted style sheet's constructor document must match ...). Rendering the
  <style> into the template creates it in the shadow root's own document and
  carries that document's litNonce.
• The plugin now only sets element.value; the component clamps, formats and
  renders. The marker position still rides on the --value custom property set
  via CSSOM, which is not governed by CSP.

Testing

• Open an RTE field using the deeplwrite preset and trigger the DeepL Write
  overlay.
• The rainbow readability bar renders and updates for both the original and the
  optimized text.
• No Adopted style sheet ... exception and no inline-style CSP violation in
  the browser console.

Follow-ups (not part of this PR)

• Merge the near-identical Core13/Core14 Edit templates into a shared Fluid
  component (they now differ only in the tab attribute).
• Dark-mode colour adaptation of the bar.

Issue: DPL-177 (subtask of DPL-122, relates to DPL-140).

[github-actions]

Documentation rendering

You can find files attached to the below linked Workflow Run URL (Logs).

Please note that files only stay for around 5 days!

Name	Link
Commit	271c4fb
Logs	https://github.com/web-vision/deepl-write/actions/runs/28866299898
Documentation	https://github.com/web-vision/deepl-write/actions/runs/28866299898/artifacts/8137791959