feat: persistent multi-share configuration (shares.json + localStorage)

Daemon side:
- internal/shares: new package reads/writes shares.json next to identity
- proto: add ShareEntry type, add_share/remove_share/list_shares IPC commands,
  shares_list event, path field on FileEntry
- netmgr: load shares.json on startup; ScanAllShares combines legacy ShareDir
  with shares.json entries (recursive walk with relative paths)
- mesh: ScanFiles callback lets manager inject multi-share scanning without
  mesh knowing about shares.json
- ipc: handle add_share, remove_share, list_shares commands

Browser side:
- ShareManager component replaces FolderPicker: shows list of named shares
  with remove/re-pick buttons; persists share records to waste_shares in
  localStorage (name, global flag, networkId)
- FolderPicker retained for internal use; Sidebar now uses ShareManager

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

internal/mesh/peer.go

@@ -201,7 +201,12 @@ func dispatchPeerMessage(msg proto.PeerMessage, from proto.PeerID, m *Mesh) {
 		m.Emit(proto.IpcMessage{Type: proto.EvtMessageReceived, Message: chat})
 
 	case proto.MsgFileListReq:
-		files := m.ScanShareDir()
+		var files []proto.FileEntry
+		if m.ScanFiles != nil {
+			files = m.ScanFiles()
+		} else {
+			files = m.ScanShareDir()
+		}
 		resp, err := json.Marshal(proto.PeerMessage{
 			Type:         proto.MsgFileListResp,
 			FileListResp: &proto.FileListResp{Files: files},