LinksMenu and SettingsMenu triggers mixed a full-colour emoji (link)
with a plain text glyph (gear) - different rendering paths mean
different, unfixable sizes/styles. Replace both with matching inline
SVG icons (feather/lucide gear + link glyphs), same viewBox/stroke/
size, so the two buttons finally share one rendering path.
Also: shared base.css never zeroed the default UA body margin, which
showed as a bg-coloured strip around the whole viewport edge on any
full-bleed header (swarm-uis .shell-header among them). Zeroed it in
the shared file so every consumer (dashboard, agent UI, swarm-ui) gets
the fix, not just swarm-ui.
Screenshot-verified at both desktop and phone widths.
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