Web application: add connection name, test button, show connected devices, remove multiple-device warning

[pbek]

Feature Request

Summary

Improve the web application settings dialog with several UX improvements:

1. Connection name setting: Add a string setting "Connection name" in the web application settings. Default value should be qownnotes-<sessionname>-<hostname>. This name is sent to the server so connected peers can see which desktop is connected.

2. Connection test button: Add a "Test connection" button in the web application settings that tests whether the WebSocket connection to the server is working and shows the result to the user.

3. Show all connected devices: In the web application settings, show a list of all devices currently connected to the same token/room on the server.

4. Remove multiple device warning: Remove the warning dialog that appears when more than 2 connections use the same token (both in QOwnNotes desktop and the web-app server). Instead, the connected devices list makes this information visible without being intrusive.

Motivation

• The current multiple-device warning is confusing, especially for users who intentionally use QOwnNotes on multiple machines or browsers.
• It's useful to know what devices are currently connected to make it easy to identify and manage sessions.
• A connection name helps identify which QOwnNotes instance is which.
• A connection test button improves discoverability and reduces support friction.

[pbek]

26.6.0

• Added Connection name setting to the web application settings, defaulting to
  qownnotes-<username>-<hostname>; the name is sent to the relay server on connect
  so other devices in the same room can identify this desktop instance
  (for #3632)
• Added a Test connection button to the web application settings that shows
  whether QOwnNotes is currently connected to the web application server
  (for #3632)
• Added a Connected devices list to the web application settings that shows
  all devices currently connected to the same token; a Refresh button requests
  an updated list from the server
  (for #3632)
• Removed the intrusive warning dialog that appeared when more than two
  connections used the same token — the connected devices list now surfaces
  this information without interrupting the user
  (for #3632)
• Changes to the web application settings (server URL, security token, enable
  toggle) now take effect immediately without restarting the application
  (for #3632)

[pbek]

26.6.1

• Fixed a crash on Windows 11 when opening the Settings dialog caused by a null
  pointer dereference on the WebSocket object inside sendRegister() and
  sendRequestConnectedDevices() — these methods were called during settings
  initialisation even when the web application feature was disabled and the
  socket was never created (for #3632)