feat(nav): per-entry inset for the v2 tab bar (no push jump)

[bmc08gt]

Summary

Fix the "content jumps on push" behaviour of the v2 tab bar without giving up the seamless, hoisted bar.

The bar stays a single persistent overlay at the app root, so tab switches are seamless — one instance, the selection pill slides, one haze source, no crossfade/remount. Only the reserved inset is made per-entry:

• New NavTabBarInsetEntryDecorator provides LocalTabBarPadding per NavEntry — the bar height on tab-home entries, zero on pushed/detail entries.
• The entry's route is resolved once, keyed by its stable contentKey (not the live backstack — a replaceAll removes the outgoing entry mid-animation, which would flip the value and, because it also moved entry.Content() between branches, remount the screen).
• So the inset is fixed for the entry's lifetime: the outgoing tab keeps its inset through a push while the incoming detail has none → no mid-transition collapse, no jump.
• NewAppContent wires the decorator in and feeds it the hoisted bar's measured height via a shared State<Dp>.

Background: an earlier attempt moved the whole bar into the decorator (per-entry rendering). That fixed the jump but made tab switches crossfade two bars and remount the screen (bar disappeared / wallet locked up). This keeps the bar hoisted and moves only the inset.

Verified on device (emulator)

• Tab switching across all four tabs incl. Scanner — bar persists, pill slides, no glitch.
• Dive-in (token info) — bar slides away, wallet keeps its inset (no jump).
• Predictive back — wallet slides in from the left, bar slides back up.
• Add Money modal — bar hidden for the modal.

./gradlew :apps:flipcash:app:compileDebugKotlin passes.