feat: signal server tag

agents-docs/features/signal-server-tag.md

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+# Signal Server Tag
+
+Users registered on a signal server can show that server's display tag on their profile card (opened by clicking their name or avatar).
+
+## Server configuration
+
+`server/data/variables.json` accepts an optional `serverTag` string. When omitted, the server falls back to its public URL built from `serverProtocol`, `serverHost`, and `serverPort`.
+
+## Health API
+
+`GET /api/health` includes `serverTag` so clients can cache the display label per configured endpoint.
+
+## WebSocket presence
+
+The client sends `homeSignalServerUrl` in `identify` messages. The signaling server echoes that value in `server_users` and `user_joined` payloads so other clients can resolve the correct tag.
+
+## Client behavior
+
+- Login and registration store `homeSignalServerUrl` on the current user.
+- Profile cards show the resolved tag beside the username in muted text.
+- Configured labels render as `#tag`; URL fallbacks render as a globe icon with the URL in a tooltip.
+- Tag resolution prefers the endpoint's cached `serverTag` from health checks, then falls back to the stored home URL.