Add keyboard message selection for screen reader users

[rezabakhshilaktasaraei]

Built on top of #30948 (the message-history action layer) - the first commit here is that PR; this one adds the second commit.

Keyboard multi-selection over history messages for screen reader users (screen-reader mode only, active only while the message list itself has keyboard focus):

• Ctrl+Space toggles selection of the focused message - the keyboard way into selection mode, from an empty selection.
• While any message stays selected, plain Space keeps toggling the focused row - mirroring how single clicks work for sighted users once selection is active - and with nothing selected it plays/pauses the focused media, as before. Escape clears the selection as usual, and Space reverts to play/pause.
• Shift+Up/Down moves the focus and extends/shrinks the selection as a contiguous range, Explorer-style (anchor-based, re-anchoring safely around the unread bar row).
• All mutations go through the same changeSelectionAsGroup paths mouse selection uses (selection cap, album atomicity, service-message exclusion), with the same repaint / top bar / announcement follow-ups.
• Clearing the last selected message used to move focus into the composer (updateTopBarSelection / listSelectionChanged); with a screen reader active the list keeps focus, so keyboard users are not yanked out of the history mid-navigation.

HistoryInner and HistoryView::ListWidget (scheduled, pinned, replies, topics) get intentionally parallel implementations.