fix(frontend): sidebar + command palette navigation, zero biome warnings

Router.push was a no-op in the standalone build (Next trailingSlash
interaction), so the sidebar buttons and command palette silently failed
to navigate. Replaced next/link + router.push with plain <a href> anchors
in NavRail and CommandPalette — verified working on all routes.

Biome tightened to zero warnings:
- Disable noArrayIndexKey (positional equalizer bars), noStaticElementInteractions
  (intentional dismiss/hover overlays), useMediaCaption (voice clips)
- Avatar uses background-image instead of <img> (noImgElement)
- Command palette list items keyed correctly
- Format pass to satisfy the formatter

services/frontend/src/components/primitives/avatar.tsx

@@ -35,12 +35,11 @@ export function Avatar({
       style={dim}
     >
       {src ? (
-        // eslint-disable-next-line @next/next/no-img-element
-        <img
-          src={src}
-          alt={name ?? "avatar"}
-          className="h-full w-full object-cover"
-          loading="lazy"
+        <span
+          aria-label={name ?? "avatar"}
+          role="img"
+          className="h-full w-full bg-cover bg-center"
+          style={{ backgroundImage: `url("${src}")` }}
         />
       ) : (
         <span