feat(browser): add mouse history navigation

[kallupx]

Description

Add mouse side-button history navigation using GTK's existing gesture support and Strata's browser history. Mouse button 8 navigates back and button 9 navigates forward when the corresponding history entry is available; unavailable actions and all other buttons remain unclaimed.

Attach the bubble-phase controller to the normal window root so navigation works over the header, breadcrumbs, sidebar, browser content, and preview while modal overlays remain isolated. The event ordering also supports routing navigation to the active pane when multi-pane browsing from #83 gets merged.

Visual evidence

N/A. This changes pointer navigation behavior without changing the interface's appearance.

How to test

1. Open Strata in Columns view and navigate through three folders: A → B → C.
2. Place the pointer over the browser content, header or breadcrumbs, sidebar, and an open preview, then use mouse buttons 8 and 9.
3. Continue using button 8 at the oldest history entry and button 9 at the newest entry.
4. Confirm left, middle, and right mouse-button behavior remains unchanged.
5. Repeat the navigation checks in Grid and Explorer views.

Expected result: Button 8 navigates backward and button 9 navigates forward from every normal window region. History boundaries are harmless, modal overlays do not trigger background navigation, and other mouse buttons behave unchanged.

Related issue

Closes #70

[l0gicgate]

Okay this is actually awesome lol, I used it just now and it feels nice as heck!