agent icon: 404 when unconfigured, client-side fallback

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.

hive-c0re/src/forge/users.rs

@@ -27,7 +27,9 @@ use crate::paths::FORGE_CORE_TOKEN as CORE_TOKEN_PATH;
 // Avatar PNG paths are resolved by `core_avatar_png_path` /
 // `config_org_avatar_png_path` below — only-used-here, so they live
 // in this module rather than the shared `hive_sh4re::assets` helpers
-// (which stay for paths every crate needs, e.g. `branding_svg`).
+// (which now hold only `prompt_template`, the one asset path every
+// crate needs — the agent icon, the last other one, moved off the
+// shared-asset model entirely; see `hive-agent::web_ui::screen`).
 
 /// `$HIVE_ASSETS_DIR/branding/hyperhive.png` — the core mark,
 /// rasterised. Not independently configurable (unlike the org avatar