hive_sh4re::assets::branding_svg() resolved a server-side default
icon at runtime from HIVE_ASSETS_DIR — the only consumer was
serve_icon(), which fell back to it whenever the agent had no
`hyperhive.icon` override. Removed both the fallback and the
function: serve_icon() now 404s when /etc/hyperhive/icon.svg is
absent, and the per-agent web UI (app.js) picks up the existing
dashboard swarm.js pattern — swap the <img> src to the
frontend-bundled /favicon.svg on load failure, guarded against
looping if the fallback itself 404s.
Updated the doc/comment claims that said the server always returns
an image (docs/web-ui/agent.md, nix/agent-modules/default.nix, the
hive-c0re/forge/users.rs comment referencing the old shared-asset
set). forge-avatar-sync and the matrix avatar sync are unaffected —
both are gated on hyperhive.icon != null and never depended on the
removed fallback.