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-tauri/tauri.conf.json

@@ -40,7 +40,10 @@
     "externalBin": ["binaries/mycelium"],
     "linux": {
       "deb": {
-        "depends": ["policykit-1"]
+        "depends": ["policykit-1"],
+        "files": {
+          "/usr/share/polkit-1/actions/tech.threefold.mycellium-ui.policy": "packaging/polkit/tech.threefold.mycellium-ui.policy"
+        }
       }
     }
   }