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/netmgr/manager.go

@@ -25,7 +25,7 @@ type Config struct {
 	MasterIdentity *crypto.Identity
 	StoreDir       string // base directory for per-network SQLite files
 	AnchorURL      string // WebSocket anchor URL used for all networks
-	ShareDir       string // shared file directory (global for now; per-network in future)
+	ShareDir       string // default share directory; overridden per network via Join or SetShareDir
 }
 
 // Network is a single joined network context.
@@ -41,6 +41,9 @@ type Network struct {
 	cancel context.CancelFunc
 }
 
+// ShareDir returns the share directory for this network (from the mesh).
+func (n *Network) ShareDir() string { return n.Mesh.ShareDir }
+
 // Manager owns all joined networks and fans out their events to IPC subscribers.
 type Manager struct {
 	cfg Config
@@ -63,7 +66,8 @@ func New(cfg Config) *Manager {
 
 // Join creates or rejoins a named network. Returns the network ID.
 // If the network is already joined the existing ID is returned immediately.
-func (mgr *Manager) Join(name string) (string, error) {
+// shareDir overrides the global default for this network; pass "" to use the default.
+func (mgr *Manager) Join(name, shareDir string) (string, error) {
 	netHash := hashNetName(name)
 	netID := netHash[:8]
 
@@ -89,7 +93,10 @@ func (mgr *Manager) Join(name string) (string, error) {
 	}
 
 	m := mesh.New(derived, st)
-	if mgr.cfg.ShareDir != "" {
+	// Per-network share dir takes precedence over the global default.
+	if shareDir != "" {
+		m.ShareDir = shareDir
+	} else if mgr.cfg.ShareDir != "" {
 		m.ShareDir = mgr.cfg.ShareDir
 	}
 	m.DownloadDir = filepath.Join(mgr.cfg.StoreDir, "downloads-"+netID_full)
@@ -130,10 +137,11 @@ func (mgr *Manager) Join(name string) (string, error) {
 	}
 
 	mgr.emit(proto.IpcMessage{
-		Type:      proto.EvtNetworkJoined,
-		NetworkID: netID,
+		Type:        proto.EvtNetworkJoined,
+		NetworkID:   netID,
 		NetworkName: name,
-		LocalPeer: peerPtr(derived.PeerInfo()),
+		LocalPeer:   peerPtr(derived.PeerInfo()),
+		ShareDir:    m.ShareDir,
 	})
 
 	return netID, nil
@@ -159,6 +167,20 @@ func (mgr *Manager) Leave(netID string) {
 	mgr.emit(proto.IpcMessage{Type: proto.EvtNetworkLeft, NetworkID: netID})
 }
 
+// SetShareDir updates the share directory for an already-joined network.
+// Changes take effect immediately for subsequent file-list requests and offers.
+func (mgr *Manager) SetShareDir(netID, path string) bool {
+	mgr.mu.RLock()
+	net, ok := mgr.networks[netID]
+	mgr.mu.RUnlock()
+	if !ok {
+		return false
+	}
+	net.Mesh.ShareDir = path
+	log.Printf("netmgr: share dir for %q set to %q", net.Name, path)
+	return true
+}
+
 // LeaveAll leaves every joined network.
 func (mgr *Manager) LeaveAll() {
 	mgr.mu.RLock()