chatdlg: replace QTextBrowser with QListView for screen reader accessibility

[mcfnord]

Replaces QTextBrowser with QListView + QStyledItemDelegate in the chat dialog so that each message is a discrete item in the OS accessibility tree.

With QTextBrowser, VoiceOver (and other screen readers that consume QAccessible) sees the entire chat history as a single text block. With QListView, each appended message becomes a QAccessible::ListItem, letting the user arrow-key through individual messages.

ChatDelegate renders each item as HTML via QTextDocument, preserving the existing rich-text formatting. Link clicks are detected in eventFilter by installing a filter on the viewport and using QTextDocument::documentLayout()->anchorAt() to hit-test the click position. Hovering over a link shows a pointing-hand cursor. Clicking routes through the existing OnAnchorClicked confirmation dialog. Right-click or Ctrl+C copies the selected message's plain text to the clipboard.

macOS VoiceOver fixes (addresses the silence reported in testing)

Two issues prevented VoiceOver from speaking on macOS:

1. Qt::AccessibleTextRole — Qt::DisplayRole stores raw HTML. VoiceOver reads this role verbatim and sees angle-bracket markup, producing silence or garbled output. Fix: QTextDocument::toPlainText() strips the tags, and the result is stored as Qt::AccessibleTextRole on each item — the role VoiceOver actually queries for the item's accessible name.

2. Correct accessibility events — Replaced the previous QAccessibleValueChangeEvent (which fired before HTML processing and before the row existed) with QAccessibleTableModelChangeEvent::RowsInserted targeting the exact new row, followed by QAccessibleValueChangeEvent carrying the plain text. This is the correct signal sequence for live-region-style auto-announcement of incoming messages on macOS.

Fixes an issue?

Addresses #3613.

Does this change need documentation?

No documentation changes needed.

Status

Builds and runs. VoiceOver on macOS should now speak individual messages when navigating the list and announce incoming messages automatically. Please test with VoiceOver — specifically: does arrowing to a message read it aloud, and does an incoming message get announced without user action?

Checklist

• I've verified that this Pull Request follows the general code principles
• I tested my code and it does what I want
• My code follows the style guide
• I waited some time after this Pull Request was opened and all GitHub checks completed without errors.

[mcfnord]

Someone please try this on a Mac using VoiceOver.