core: skip network-stack rebuild when SetNetworkMode is a no-op

The GUI re-sends the saved network mode as part of its startup config-sync.
The SetNetworkMode handler unconditionally tore down + rebuilt the iroh
endpoint whenever idle, so every launch rebuilt the freshly-built stack for
an identical posture — a needless ~1s teardown+rebuild bounce visible in the
logs on both Linux and Windows/Wine (the 'start core loop -> shut down network
stack ~1s later' pattern from the Wine spike). Guard the rebuild on an actual
mode change; a real change still rebuilds exactly as before.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/core/mod.rs

@@ -1816,6 +1816,14 @@ async fn run_core_loop(
             }
 
             CoreCommand::SetNetworkMode(mode) => {
+                // Skip when the posture is unchanged. The GUI re-sends the saved
+                // network mode as part of its startup config-sync, and that mode
+                // usually already matches the freshly-built stack — rebuilding the
+                // iroh endpoint for an identical posture just churns the network
+                // and adds a needless ~1s teardown+rebuild bounce at every launch
+                // (seen on both Linux and Windows/Wine). A real change still
+                // rebuilds exactly as before.
+                if mode != network_mode {
                     network_mode = mode;
                     // Rebuild the persistent stack to the new posture immediately if
                     // idle; if a call is active, defer to the next Leave/Join so the
@@ -1829,6 +1837,7 @@ async fn run_core_loop(
                         net_rebuild_pending = true;
                     }
                 }
+            }
 
             CoreCommand::RegenerateIdentity => {
                 // Mint + persist a fresh identity, discarding the old one. The