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/identity.rs

@@ -257,11 +257,10 @@ impl IdentityManager {
             anyhow::bail!("Identity {} not found", &target_hex[..12]);
         }
 
-        // Shutdown current node if running
+        // Shutdown current node before switching
         if let Some(ref node) = self.active_node {
-            info!(old = hex::encode(node.node_id), new = target_hex, "Switching identity");
-            // Node::shutdown would go here once implemented
-            // For now, dropping the Arc triggers cleanup when last reference is released
+            info!(old = hex::encode(node.node_id), new = target_hex, "Switching identity — shutting down old node");
+            node.network.shutdown_ref().await;
         }
         self.active_node = None;
         self.active_id = None;

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(