fix: Make toolbar sticky when editing markdown content

[sierragolflima]

Description

When editing a markdown file in MeshWeaver, the back/done button in the toolbar only responds to clicks when the user has scrolled to the top of the document. This forces users to scroll up before they can save or exit edit mode.

Current Behavior

The "Done" button in the toolbar only works after scrolling to the top of the document.

Expected Behavior

The toolbar with Save/Done buttons should remain accessible and clickable regardless of scroll position.

Files to Modify

• src/MeshWeaver.Blazor/Articles/ArticleHeaderEditor.razor (lines 9-12)
• src/MeshWeaver.Blazor/Articles/ArticleHeaderEditor.razor.cs (lines 54-57)

Implementation Hint

Apply sticky positioning to the <FluentToolbar>:

fluent-toolbar {
    position: sticky;
    top: 0;
    z-index: 100;
    background: var(--neutral-layer-1);
}

Acceptance Criteria

• Toolbar remains visible when scrolling in edit mode
• Save and Done buttons are clickable at any scroll position
• Toolbar has appropriate background to not blend with content