First-run chooser, node shutdown on switch, file picker, export path fix

- First-run: show Start Fresh / Import chooser only when single auto-created
  identity with no profile (not on every boot without a display name).
- Identity switch: shut down old node's endpoint before starting new one.
  Fixes lockup after multiple switches (zombie background tasks).
- File picker: native Browse buttons on export (folder) and import (ZIP file)
  via tauri-plugin-dialog.
- Export path: resolve relative paths against home dir (was using process cwd).
- Lightbox close: only close on overlay/image click, not inner form content.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/core/src/network.rs

@@ -2335,6 +2335,14 @@ impl Network {
         Ok(())
     }
 
+    /// Shutdown via Arc reference — closes the endpoint, causing all background tasks to exit.
+    pub async fn shutdown_ref(&self) {
+        if let Some(ref mapping) = self.upnp_mapping {
+            crate::upnp::remove_upnp_mapping(mapping.external_addr.port()).await;
+        }
+        self.endpoint.close().await;
+    }
+
     /// Propagate an engagement diff to all downstream holders of a post (CDN tree).
     /// Excludes the sender to avoid loops.
     pub async fn propagate_engagement_diff(