Fix broken action buttons and add delete confirmation modal

- shares.js, users.js: JSON.stringify() inside onclick="..." attributes
  produced unescaped " chars that broke HTML attribute parsing — all
  dynamic onclick JSON now passed through esc() so the browser decodes
  " back to " before executing the handler
- Replace browser confirm() dialogs with a styled Bootstrap modal
  (#deleteModal) for share and user deletion — reusable across all tabs
- users.js: rename shadow variable 'confirm' → 'pwConfirm' to avoid
  shadowing the now-removed confirm() call
- main.js: auto-refresh now reloads shares and users every 30 s,
  not just the dashboard

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

static/js/main.js

@@ -1,4 +1,4 @@
 loadDashboard();
 loadShares();
 loadUsers();
-setInterval(loadDashboard, 30_000);
+setInterval(() => { loadDashboard(); loadShares(); loadUsers(); }, 30_000);