fix: Make edit menu button sticky when scrolling in markdown view

[sierragolflima]

Description

When viewing a markdown file in MeshWeaver, the edit menu button (ellipsis) scrolls out of view when the user scrolls down the document. Users cannot access edit options without scrolling back to the top.

Current Behavior

The edit menu button disappears from view when scrolling down a long markdown document.

Expected Behavior

The edit menu button should remain visible (sticky) at the top-right corner regardless of scroll position.

Files to Modify

• src/MeshWeaver.Blazor/Articles/ArticleHeaderDisplay.razor (lines 36-53)
• src/MeshWeaver.Blazor/Articles/ArticleHeaderDisplay.razor.css

Implementation Hint

The <FluentButton id="editMenuButton"> and <FluentMenu> need to be wrapped in a sticky container:

.edit-menu-container {
    position: sticky;
    top: 0;
    z-index: 100;
}

Acceptance Criteria

• Edit menu button stays visible when scrolling in markdown view
• Menu dropdown still anchors correctly to the button
• Button doesn't overlap content awkwardly