P5: settings, persistence, polkit packaging, README

Backend
- regenerate_identity command stops the daemon, deletes
  priv_key.bin, leaves the user to restart for a fresh identity;
  falls back to the canonical XDG path when sidecar.key_path()
  isn't populated yet
- tauri.conf.json ships the polkit policy via deb.files mapping;
  src-tauri/packaging/polkit/tech.threefold.mycellium-ui.policy
  declares the spawn action with auth_admin_keep so the dialog
  appears once per session

Frontend
- config store persists SidecarConfig (peers, tunName, noTun)
  through tauri-plugin-store; App.vue reads it and forwards to
  start_daemon, replacing the hard-coded defaults
- Settings view: daemon-config form, identity panel with the
  destructive regenerate button, sidecar log viewer, About
- README rewritten end-to-end: HTTP-loopback architecture, polkit
  install path, build commands, verification matrix, and a
  honest "known limitations" section

src/stores/node.ts

@@ -1,6 +1,6 @@
 import { defineStore } from "pinia";
 import { ref } from "vue";
-import { api, type DaemonStatus, type NodeInfo } from "@/lib/api";
+import { api, type DaemonStatus, type NodeInfo, type SidecarConfig } from "@/lib/api";
 import { Events, on } from "@/lib/events";
 
 export type Phase = "idle" | "starting" | "ready" | "error";
@@ -41,11 +41,11 @@ export const useNodeStore = defineStore("node", () => {
     }
   }
 
-  async function start() {
+  async function start(config?: SidecarConfig) {
     phase.value = "starting";
     error.value = null;
     try {
-      const s = await api.startDaemon();
+      const s = await api.startDaemon(config);
       status.value = s;
       info.value = await api.nodeInfo();
       phase.value = "ready";