fix(v2): host the tab bar as a bottom overlay, not a scaffold bottomBar

[bmc08gt]

Problem

As a Material2 Scaffold bottomBar, the v2 tab bar couldn't be correct either way:

• Hiding it for a BottomBar modal collapsed the scaffold's bottom slot, which grew the body viewport and re-laid-out the content beneath — visibly re-fanning the wallet's collapsing card stack when Add Money opened.
• Keeping it visible drew the bar on top of the modal (a Scaffold bottomBar is painted over the content, where the modal lives), so z-order couldn't cover it.

Fix

Host AppNavigationBar as a bottom overlay over full-height nav content — which is exactly what its own doc always described ("a full-size, touch-transparent overlay pinned to the bottom"):

• The body no longer resizes when the bar hides, so hiding it for a modal (restored) or a bill is free → no jump.
• The modal renders over the now-absent bar → no bar over the modal.
• LocalTabBarPadding is provided from the bar's tallest measured height, latched and gated to tab homes, so a modal-driven hide doesn't shrink the inset.
• Content now scrolls edge-to-edge under the frosted bar, which is what the Haze blur wants.

Test Plan

• App compiles; installed on emulator.
• On device: open Add Money on the wallet → the tab bar is not drawn over the modal, and the card stack stays collapsed. Close it → everything returns cleanly. Tab switching, the frosted blur, and pushed detail screens (no tab bar, no phantom bottom gap) all look right; a presented bill still hides the bar.