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/src/commands.rs

@@ -247,3 +247,52 @@ pub async fn topic_forward_remove(state: State<'_, AppState>, topic_b64: String)
 pub async fn lookup_pubkey(state: State<'_, AppState>, ip: String) -> AppResult<PubkeyLookup> {
     require_client(&state)?.lookup_pubkey(&ip).await
 }
+
+// ─── Identity ────────────────────────────────────────────────────────────────
+
+/// Stops the daemon (if running), removes the saved private key file, and
+/// returns the daemon to the idle state. The caller restarts the daemon
+/// to provoke regeneration.
+#[tauri::command]
+pub async fn regenerate_identity(state: State<'_, AppState>) -> AppResult<()> {
+    state.poller.stop();
+    let key_path = state.sidecar.key_path();
+    state.sidecar.stop().await;
+
+    if let Some(path) = key_path {
+        if path.exists() {
+            std::fs::remove_file(&path).map_err(AppError::from)?;
+        }
+    } else {
+        // Sidecar never started; resolve the canonical path via app_data_dir.
+        if let Some(p) = default_key_path() {
+            if p.exists() {
+                std::fs::remove_file(&p).map_err(AppError::from)?;
+            }
+        }
+    }
+    Ok(())
+}
+
+fn default_key_path() -> Option<std::path::PathBuf> {
+    // Best-effort: fall back to the same XDG location the sidecar uses.
+    dirs_like_app_data().ok().map(|d| d.join("priv_key.bin"))
+}
+
+fn dirs_like_app_data() -> std::io::Result<std::path::PathBuf> {
+    // We can't reach the AppHandle here, so we mirror Tauri's path:
+    // $XDG_DATA_HOME/<identifier>/ or $HOME/.local/share/<identifier>/.
+    let identifier = "tech.threefold.mycellium-ui";
+    if let Ok(d) = std::env::var("XDG_DATA_HOME") {
+        return Ok(std::path::PathBuf::from(d).join(identifier));
+    }
+    if let Ok(home) = std::env::var("HOME") {
+        return Ok(std::path::PathBuf::from(home)
+            .join(".local/share")
+            .join(identifier));
+    }
+    Err(std::io::Error::new(
+        std::io::ErrorKind::NotFound,
+        "no app data dir",
+    ))
+}