feat(home): open the account switcher on a long-press of the You tab

[bmc08gt]

Long-pressing the You tab now opens the account switcher, as iOS does in HomeTabView.handleLongPress(on:).

• Only the You tab takes a long-press. Scan, Wallet and Chat don't.
• The gate is the one that shows Switch Accounts in Advanced Features: the beta-flags override is on, or the user is staff. HomeViewModel.canSwitchAccounts combines the two, so both entry points agree.
• With the gate closed, NavigationBar gets a null onYouTabLongClick, so the tab has no long-click at all: no haptic, and a long hold selects the tab like a tap does.
• With the gate open, the existing AppRoute.Menu.AccountSelection screen opens through openAsSheet, and its app bar shows Close. Choosing an account uses the existing logout-and-switch path in App.kt, which replaces the whole stack, sheet included.
• A tap still selects the tab. The indication stays null, so no ripple is added or lost.

The haptic is the one combinedClickable fires (HapticFeedbackType.LongPress), not a medium impact like iOS's.

Tests:

• NavigationBarLongPressTest (core-ui, Compose): gate closed means no haptic and no callback; gate open fires both; a tap still selects; the other tabs take no long-press.
• YouTabLongClickTest (app, Robolectric): gate closed gives no handler; gate open pushes AccountSelection as a sheet. This adds robolectric to the app's test dependencies and :shared:userflags to its implementation dependencies.