docs: help icon in the pixel-art editor toolbar

Adds an ⓘ at the start of the pixel editor's tool bar (left of the
pencil/eraser/fill tool group) that opens the Pixel art docs page.
Pixel mode runs through its own bespoke toolbar (not the standard
left-sidebar fieldset flow), so the editor bar is the right home
for the icon.

Co-Authored-By: Claude Opus 4.7 <[email protected]>

src/index.html

@@ -4712,6 +4712,7 @@
 
         <div id="pixelEditorView">
           <div class="pixelEditorBar">
+            <span class="docsHelpIcon" data-help="pixelart" role="button" aria-label="Open help: Pixel art" title="Open help: Pixel art" style="margin-right:6px">ⓘ</span>
             <div class="pixelToolGroup">
               <button type="button" class="pixelToolBtn active" data-ptool="pencil" title="Pencil">✏</button>
               <button type="button" class="pixelToolBtn" data-ptool="eraser" title="Eraser">⌫</button>