Replace legacy MobileBottomNav with the new 4-destination MobileTabBar
that mirrors the desktop IconRail (Assistant, Studio, Projects,
Settings). Adds volt active states with a 2px bar above the icon,
safe-area bottom padding, and scroll-hide wiring via the existing
mobileNavVisible handler in Layout.
Adapts Phase 9 HistorySheet and MemorySheet to a full-screen variant
below 768px via useMediaQuery, and makes Phase 11 BuilderTabStrip
horizontally scrollable with scroll-snap and edge fade on mobile.
Light TopStrip polish (tighter padding, truncating breadcrumb)
completes the mobile frame.
PromoteTransition mobile variant is deferred — Phase 12 had not yet
landed when Phase 15 started; the controller can add the mobile
branch when merging.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
4623c8aea0