Keyboard access: regions, message actions, call and gallery focus - #3450

The keyboard half of the UnigramPlus pass: things the add-on had to invent because the app has no key for them.

What it does

• F6 / Shift+F6 cycle the regions of a window — folder tabs, chat list, messages, composer, and the search box. The regions are the elements already marked with a landmark in XAML; the walk stops at each one, so the message list's realized children are never visited. Works in a standalone chat window too.
• Reply, edit and forward from the focused message: Enter, Ctrl+E, Ctrl+Shift+F, alongside the Delete and Ctrl+C that were already there. Each runs the same predicate the context menu builds itself from, so nothing becomes possible that the menu wouldn't offer.
• F3 / Shift+F3 walk chat search results, in the direction the search runs.
• Ctrl+Alt+M jumps to the next unread mention — the floating button's action, which had no key.
• Ctrl+PageUp/PageDown toggle the call camera and microphone. These commands were defined and bound but nothing handled them: a call window hosts no navigation service, so shortcuts never reached it. WindowContext now raises one for the window's own content once no frame has taken it. The new state is announced, since the shortcut leaves focus wherever it was.
• Closing the gallery hands focus back to the message it was opened from. ModalPopup's save/restore — including the ordering that makes it work, restore before the popup closes — is now a shared FocusScope that both use.

Choices you may want to change

• Ctrl+E and Ctrl+Shift+F are both chat search. Scoped as agreed: they act on the message only when a message has focus, and fall through to chat search everywhere else. It does mean chat search is unreachable from a focused message — say the word and I'll move edit and forward to keys that shadow nothing.
• Enter was free in the message list (IsItemClickEnabled="False"), so nothing was displaced.
• Every new command is remappable and shows under a "Navigation" heading in Settings > Shortcuts.

Not done, and why

• Focus on open: already handled, already lands on the chat list.
• A key for the composer: F6 reaches it; the add-on's Alt+D also toggles back to where you were, which F6 does not.
• Enter on a settings category: those are buttons, so it already works.
• Chat-level mention navigation (the add-on's Ctrl+Alt+Up/Down walks chats with mentions): the app has no such collection; Ctrl+Alt+M is the in-chat one.

Not built, not tested with a reader — the call and gallery paths especially want a run.