The mouse back/forward buttons (XButton1/XButton2) now walk the user's
browsing history, browser-style: folders visited in the library and
strategies opened. Back from an open strategy returns to the library
through the existing unsaved-changes guard; forward re-opens it (or
swaps strategies in place, like the quick switcher).

A top-level Listener above the Navigator catches the side-button
presses, and a small NavigatorObserver tracks the route stack so the
buttons never navigate underneath dialogs; on auxiliary screens like
the fullscreen image viewer, back simply closes them. Stale history
entries (deleted folders/strategies) are skipped silently.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>