docs: replace static screenshot with animated demo + still

Swap the linked github-user-content screenshot in the README for a
short animated GIF recorded locally: user types four lines, then
clicks the Center, Right, and Justify toolbar buttons ep_align adds,
so the visual effect of each alignment is visible with the cursor
moving to the button being pressed.

Also add demo.png — the same frame as the tail of the GIF — for any
reader or registry that doesn't render animated GIFs.

Generated with the plugins/_demo-gif/ tooling (Playwright record with
injected cursor overlay + diff-based auto-crop that unions click
bounding boxes + ffmpeg palette GIF).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: JohnMcLear

README.md

@@ -2,7 +2,7 @@
 
 Adds text alignment buttons (left, center, justify, right) to the Etherpad toolbar, with HTML export support.
 
-![Screenshot](https://user-images.githubusercontent.com/220864/107214131-5c3dd600-6a01-11eb-82d9-b2d67ec8ae93.png)
+![Demo](demo.gif)
 
 ## Install