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.

docs/web-ui/agent.md

@@ -383,10 +383,10 @@ shaped).
   earliest recorded turn with an adaptive bucket width.
 - `GET /icon` — agent's icon as `image/svg+xml`. Returns
   `/etc/hyperhive/icon.svg` (set via `hyperhive.icon` in `agent.nix`)
-  when present, otherwise the bundled default hyperhive logo. Always
-  returns an image — consumers (dashboard container row, per-agent
-  favicon) can hit `/icon` unconditionally without probing for a
-  custom config.
+  when present, otherwise **404** — there is no server-side default.
+  Consumers (dashboard container row, this page's own header icon)
+  hit `/icon` optimistically and fall back client-side on load failure
+  to the frontend-bundled `/favicon.svg` rather than probing first.
 - `GET /events/history` — replay buffer for the terminal.
 - `GET /screen` — VNC viewer page (minimal RFB-over-WebSocket
   renderer — deliberately thin, just enough to display the