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>

internal/ipc/ipc.go

@@ -117,12 +117,12 @@ func handleClient(conn net.Conn, mgr *netmgr.Manager) {
 				send(errMsg("join_network: network_name is required"))
 				continue
 			}
-			netID, err := mgr.Join(cmd.NetworkName)
+			netID, err := mgr.Join(cmd.NetworkName, cmd.ShareDir)
 			if err != nil {
 				send(errMsg(fmt.Sprintf("join_network: %v", err)))
 				continue
 			}
-			// network_joined event is emitted by Manager.Join; nothing extra needed.
+			// network_joined event (with share_dir) is emitted by Manager.Join.
 			_ = netID
 
 		case proto.CmdLeaveNetwork:
@@ -246,6 +246,17 @@ func handleClient(conn net.Conn, mgr *netmgr.Manager) {
 				InviteString: inv,
 			})
 
+		case proto.CmdSetShareDir:
+			n := mgr.Resolve(cmd.NetworkID)
+			if n == nil {
+				send(errMsg("set_share_dir: not joined to any network"))
+				continue
+			}
+			if !mgr.SetShareDir(n.ID, cmd.Path) {
+				send(errMsg("set_share_dir: network not found"))
+				continue
+			}
+
 		case proto.CmdSendFile:
 			n := mgr.Resolve(cmd.NetworkID)
 			if n == nil {
@@ -292,6 +303,8 @@ func stateSnapshot(mgr *netmgr.Manager) proto.IpcMessage {
 			NetworkID:   n.ID,
 			NetworkName: n.Name,
 			LocalPeer:   &pi,
+			ShareDir:    n.Mesh.ShareDir,
+			DownloadDir: n.Mesh.DownloadDir,
 		})
 	}
 	msg.Networks = netInfos
@@ -322,7 +335,7 @@ func randomHex(n int) string {
 // It joins the network before the IPC listener starts accepting clients.
 func AutoJoin(ctx context.Context, mgr *netmgr.Manager, networkName string) {
 	_ = ctx // Manager owns the context internally
-	if _, err := mgr.Join(networkName); err != nil {
+	if _, err := mgr.Join(networkName, ""); err != nil {
 		log.Printf("ipc: auto-join %q failed: %v", networkName, err)
 	}
 }