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/components/InfoCard.vue

@@ -0,0 +1,32 @@
+<script setup lang="ts">
+import { Copy, Check } from "lucide-vue-next";
+
+defineProps<{
+  label: string;
+  value: string;
+  copied?: boolean;
+}>();
+
+defineEmits<{ (e: "copy"): void }>();
+</script>
+
+<template>
+  <div
+    class="flex items-start justify-between gap-4 rounded-lg border border-border bg-card p-4"
+  >
+    <div class="min-w-0 flex-1">
+      <div class="text-xs uppercase tracking-wide text-muted-foreground">
+        {{ label }}
+      </div>
+      <div class="mt-1 break-all font-mono text-sm">{{ value }}</div>
+    </div>
+    <button
+      class="shrink-0 rounded-md border border-border p-2 hover:bg-secondary"
+      :title="copied ? 'Copied' : 'Copy'"
+      @click="$emit('copy')"
+    >
+      <Check v-if="copied" class="h-4 w-4 text-emerald-500" />
+      <Copy v-else class="h-4 w-4" />
+    </button>
+  </div>
+</template>