swarm-ui: fix inconsistent header icons + body edge gap

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.

frontend/packages/shared/src/base.css

@@ -6,6 +6,16 @@
    top of this and must NOT redeclare the colour variables; `theme.css`
    is the one source of truth. */
 body {
+  /* The default UA stylesheet gives `body` an 8px margin — every page
+     using this shared base had it as a `--bg`-coloured strip visible
+     around the whole viewport edge, e.g. left/right of any full-bleed
+     header (swarm-ui's `.shell-header`, `--bg-elev`, doesn't actually
+     reach the real viewport edge without this). Reported live as
+     "small bg colored gap left and right of the header" (reported live
+     by mara).
+     Zeroed here since every page linking this file wants full-bleed
+     chrome, not a browser default nobody asked for. */
+  margin: 0;
   background: var(--bg);
   color: var(--fg);
   font-family: "JetBrains Mono", "Fira Code", "Cascadia Code", "Source Code Pro", monospace;