Support screen reader focus and activate on chat list rows

[rezabakhshilaktasaraei]

The chat list rows are painted/virtual (no real QWidget), so a screen reader couldn't move focus to a row or activate one. With the action interface added in desktop-app/lib_ui#303, those requests now reach InnerWidget:

Move focus (e.g. NVDA+Shift+Numpad Minus) -> accessibilityChildSetFocus(int index):

• hops to the main thread (UIA invokes provider actions on a background thread);
• moves the list's internal selection to that row (Default and Filtered states);
• scrolls it into view and reports it via the existing announceSelectedFocus() path.

Activate (e.g. NVDA+Numpad Enter / UIA Invoke) -> accessibilityChildActivate(int index):

• on the main thread, selects that row and opens the chat via chooseRow().

The selection logic shared by both is extracted into selectChildByIndex(), and the filtered-mode scroll is extracted from selectSkip() into a reusable scrollToFilteredSelected().

Depends on desktop-app/lib_ui#303 (adds the accessibilityChildSetFocus / accessibilityChildActivate virtuals); CI will not compile until that is merged and the submodule is updated.