P1: sidecar lifecycle and HTTP bridge

Backend
- sidecar.rs supervises the bundled `mycelium` binary launched via
  pkexec; locates it in resource_dir or CARGO_MANIFEST_DIR/binaries
  matching $TAURI_ENV_TARGET_TRIPLE
- ephemeral port via portpicker, key + config persisted in
  app_data_dir, kill_on_drop with explicit start_kill on stop
- health-check loop calls /api/v1/admin until 2xx (timeout 20s);
  emits sidecar://ready and sidecar://exited
- 500-line ring buffer of stdout/stderr surfaced via sidecar_logs
  command for the upcoming Settings page
- elevation::is_auth_failure(126|127) maps pkexec cancel to a
  dedicated AppError variant
- AppError uses thiserror, Serialize impl renders messages as
  plain strings for the JS side

Frontend
- typed `api` wrapper around invoke() in src/lib/api.ts
- node store (Pinia) bootstraps on mount, listens on
  sidecar://ready and sidecar://exited
- StartupOverlay covers the whole window for idle/starting/error
  phases; sidebar status dot + start/stop button
- Status view renders subnet, pubkey, api endpoint and key path
  with one-click clipboard copy

src/stores/node.ts

@@ -0,0 +1,100 @@
+import { defineStore } from "pinia";
+import { ref } from "vue";
+import { api, type DaemonStatus, type NodeInfo } from "@/lib/api";
+import { Events, on } from "@/lib/events";
+
+export type Phase = "idle" | "starting" | "ready" | "error";
+
+export const useNodeStore = defineStore("node", () => {
+  const phase = ref<Phase>("idle");
+  const status = ref<DaemonStatus | null>(null);
+  const info = ref<NodeInfo | null>(null);
+  const error = ref<string | null>(null);
+
+  let exitedUnlisten: (() => void) | null = null;
+  let readyUnlisten: (() => void) | null = null;
+
+  async function bootstrap() {
+    if (!exitedUnlisten) {
+      exitedUnlisten = await on<number>(Events.SidecarExited, (e) => {
+        error.value = `daemon exited (code ${e.payload})`;
+        phase.value = "error";
+        status.value = { running: false, apiUrl: null, keyPath: null, configPath: null };
+        info.value = null;
+      });
+    }
+    if (!readyUnlisten) {
+      readyUnlisten = await on<string>(Events.SidecarReady, async () => {
+        await refresh();
+      });
+    }
+
+    const cur = await api.daemonStatus();
+    status.value = cur;
+    if (cur.running) {
+      phase.value = "ready";
+      try {
+        info.value = await api.nodeInfo();
+      } catch (e) {
+        error.value = String(e);
+      }
+    }
+  }
+
+  async function start() {
+    phase.value = "starting";
+    error.value = null;
+    try {
+      const s = await api.startDaemon();
+      status.value = s;
+      info.value = await api.nodeInfo();
+      phase.value = "ready";
+    } catch (e) {
+      error.value = String(e);
+      phase.value = "error";
+      throw e;
+    }
+  }
+
+  async function stop() {
+    try {
+      const s = await api.stopDaemon();
+      status.value = s;
+      info.value = null;
+      phase.value = "idle";
+    } catch (e) {
+      error.value = String(e);
+      throw e;
+    }
+  }
+
+  async function refresh() {
+    status.value = await api.daemonStatus();
+    if (status.value.running) {
+      info.value = await api.nodeInfo();
+      phase.value = "ready";
+    } else {
+      info.value = null;
+      phase.value = "idle";
+    }
+  }
+
+  function dispose() {
+    exitedUnlisten?.();
+    readyUnlisten?.();
+    exitedUnlisten = null;
+    readyUnlisten = null;
+  }
+
+  return {
+    phase,
+    status,
+    info,
+    error,
+    bootstrap,
+    start,
+    stop,
+    refresh,
+    dispose,
+  };
+});