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/shares/shares.go

@@ -0,0 +1,99 @@
+// Package shares manages the persistent multi-share configuration stored in shares.json.
+// Shares are additive to the existing single-dir ShareDir mechanism.
+package shares
+
+import (
+	"encoding/json"
+	"os"
+	"path/filepath"
+	"sync"
+)
+
+// Share describes one shared directory entry.
+type Share struct {
+	Path     string   `json:"path"`
+	Networks []string `json:"networks"` // ["*"] = global (all networks); otherwise list of network IDs
+}
+
+// Store manages the shares.json file.
+type Store struct {
+	mu     sync.RWMutex
+	path   string
+	shares []Share
+}
+
+// Load reads shares.json from dataDir. Returns an empty store if the file doesn't exist.
+func Load(dataDir string) (*Store, error) {
+	s := &Store{path: filepath.Join(dataDir, "shares.json")}
+	data, err := os.ReadFile(s.path)
+	if os.IsNotExist(err) {
+		return s, nil
+	}
+	if err != nil {
+		return nil, err
+	}
+	if err := json.Unmarshal(data, &s.shares); err != nil {
+		return nil, err
+	}
+	return s, nil
+}
+
+func (s *Store) save() error {
+	data, err := json.MarshalIndent(s.shares, "", "  ")
+	if err != nil {
+		return err
+	}
+	return os.WriteFile(s.path, data, 0600)
+}
+
+// All returns a copy of all shares.
+func (s *Store) All() []Share {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+	out := make([]Share, len(s.shares))
+	copy(out, s.shares)
+	return out
+}
+
+// ForNetwork returns all shares visible on networkID (global + network-specific).
+func (s *Store) ForNetwork(networkID string) []Share {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+	var out []Share
+	for _, sh := range s.shares {
+		for _, n := range sh.Networks {
+			if n == "*" || n == networkID {
+				out = append(out, sh)
+				break
+			}
+		}
+	}
+	return out
+}
+
+// Add adds a share (no-op if path already present). Persists immediately.
+func (s *Store) Add(sh Share) error {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	for i, existing := range s.shares {
+		if existing.Path == sh.Path {
+			s.shares[i].Networks = sh.Networks
+			return s.save()
+		}
+	}
+	s.shares = append(s.shares, sh)
+	return s.save()
+}
+
+// Remove removes the share with the given path. Persists immediately.
+func (s *Store) Remove(path string) error {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	for i, sh := range s.shares {
+		if sh.Path == path {
+			s.shares = append(s.shares[:i], s.shares[i+1:]...)
+			return s.save()
+		}
+	}
+	return nil
+}