feat(electrobun): multi-machine relay + OTEL telemetry

Multi-machine relay:
- relay-client.ts: WebSocket client for agor-relay with token auth,
  exponential backoff (1s-30s), TCP probe, heartbeat (15s ping)
- machines-store.svelte.ts: remote machine state tracking
- RemoteMachinesSettings.svelte: machine list, add/connect/disconnect UI
- 7 RPC types (remote.connect/disconnect/list/send/status + events)

Telemetry:
- telemetry.ts: OTEL spans + OTLP/HTTP export to Tempo,
  controlled by AGOR_OTLP_ENDPOINT env var
- telemetry-bridge.ts: tel.info/warn/error frontend convenience API
- telemetry.log RPC for frontend→Bun tracing

ui-electrobun/src/mainview/telemetry-bridge.ts

@@ -0,0 +1,45 @@
+/**
+ * Frontend telemetry bridge.
+ *
+ * Provides tel.info(), tel.warn(), tel.error() convenience methods that
+ * forward structured log events to the Bun process via RPC for tracing.
+ * No browser OTEL SDK needed (WebKitGTK incompatible).
+ */
+
+import { appRpc } from "./rpc.ts";
+
+type LogLevel = "info" | "warn" | "error";
+type Attributes = Record<string, string | number | boolean>;
+
+function sendLog(level: LogLevel, message: string, attributes?: Attributes): void {
+  try {
+    appRpc?.request["telemetry.log"]({
+      level,
+      message,
+      attributes: attributes ?? {},
+    }).catch((err: unknown) => {
+      // Best-effort — never block the caller
+      console.warn("[tel-bridge] RPC failed:", err);
+    });
+  } catch {
+    // RPC not yet initialized — swallow silently
+  }
+}
+
+/** Frontend telemetry API. All calls are fire-and-forget. */
+export const tel = {
+  /** Log an informational event. */
+  info(message: string, attributes?: Attributes): void {
+    sendLog("info", message, attributes);
+  },
+
+  /** Log a warning event. */
+  warn(message: string, attributes?: Attributes): void {
+    sendLog("warn", message, attributes);
+  },
+
+  /** Log an error event. */
+  error(message: string, attributes?: Attributes): void {
+    sendLog("error", message, attributes);
+  },
+} as const;