Add WebDAV support via Apache mod_dav

Backend:
- WebDAV config managed at /etc/apache2/conf-available/micronas-webdav.conf
  with BEGIN/END markers per share; reload apache2 on every write
- htpasswd file at /etc/micronas-webdav.passwd synced automatically on
  create_user, change_password, delete_user
- Full CRUD: POST/PATCH/DELETE /api/share/webdav[/<name>]
- _validate_webdav_users(): space-separated, no @group (not supported by Apache)
- Fix: _validate_users() now allows @groupname for SMB (was incorrectly rejected)
- Dashboard: apache2 status + restart button; webdav shares in /api/dashboard

Frontend:
- WebDAV tab: create form + table with clickable access URLs
- Edit modal supports 'webdav' type (name, path, users, permissions section)
- badge-webdav CSS class (green tint) in app.css
- dashboard.js renders badge-<type> dynamically (smb/nfs/webdav)

Docs: README WebDAV mount guide (Linux davfs2, Windows, Mac), PLANNING phase 4

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

static/css/app.css

@@ -50,8 +50,9 @@ body {
 .progress { background: var(--surf2); border-radius: 4px; }
 
 /* Badges */
-.badge-smb { background: #1f3a5f; color: #58a6ff; font-size: .72rem; }
-.badge-nfs { background: #2d1f5f; color: #a78bfa; font-size: .72rem; }
+.badge-smb    { background: #1f3a5f; color: #58a6ff; font-size: .72rem; }
+.badge-nfs    { background: #2d1f5f; color: #a78bfa; font-size: .72rem; }
+.badge-webdav { background: #1f3a2f; color: #3fb950; font-size: .72rem; }
 
 /* Modal */
 .modal-content { background: var(--surf); border: 1px solid var(--border); }