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/mesh.go

@@ -29,6 +29,9 @@ type Mesh struct {
 	Store       *store.Store // may be nil if persistence is disabled
 	ShareDir    string       // directory whose contents are shared with peers; "" = no sharing
 	DownloadDir string       // directory where received files are saved
+	// ScanFiles overrides ScanShareDir when set — allows the manager to inject
+	// multi-share scanning without the mesh needing to know about shares.json.
+	ScanFiles func() []proto.FileEntry
 
 	mu    sync.RWMutex
 	peers map[proto.PeerID]*PeerConn