feat: per-network share directories + isolation test

Each network now carries its own share dir, set at join_network time via
optional share_dir field or updated live with set_share_dir. The global
-share-dir daemon flag becomes a fallback default.

- proto: add ShareDir/DownloadDir to NetworkInfo and IpcMessage
- netmgr: Join accepts shareDir override; SetShareDir updates live
- ipc: wire join_network share_dir and set_share_dir command
- daemon: remove -share-dir from auto-join path (pass "" for default)
- test-network.sh: per-network join with share_dir; isolation verification
  section confirms alice/friends and alice/work share dirs are independent
- test-tui.sh: join_network with share_dir; peer IDs resolved after join

All tests pass: YAW/2.1 FS, share isolation, file transfer, persistence.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cmd/daemon/main.go

@@ -50,7 +50,7 @@ func main() {
 	})
 
 	if autoJoinNetwork != "" {
-		if _, err := mgr.Join(autoJoinNetwork); err != nil {
+		if _, err := mgr.Join(autoJoinNetwork, ""); err != nil {
 			log.Fatalf("auto-join: %v", err)
 		}
 	}