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>

web/src/components/Sidebar.tsx

@@ -1,7 +1,7 @@
 import { useState } from 'react'
 import { useWaste } from '../store'
 import type { PeerStatus } from '../store'
-import { FolderPicker } from './FolderPicker'
+import { ShareManager } from './ShareManager'
 import { Transfers } from './Transfers'
 
 function makeYawCard(id: string, alias: string): string {
@@ -124,7 +124,7 @@ export function Sidebar() {
       {adapterMode === 'browser' && (
         <div className="sidebar-section">
           <span className="sidebar-label">Sharing</span>
-          <div style={{ padding: '4px 12px' }}><FolderPicker /></div>
+          <div style={{ padding: '4px 12px' }}><ShareManager /></div>
         </div>
       )}