Implement responsive mobile navigation with bottom sheet

[AlienScroll78]

Overview

Replace the current top navigation with a mobile-optimized bottom navigation bar on small screens, using a bottom-sheet pattern for secondary actions as documented in docs/mobile-claim-bottom-sheet.md.

Acceptance Criteria

• Bottom navigation bar on viewports < 768px
• Tabs: Dashboard, Create, History, Settings
• Claim bottom sheet slides up on stream card tap
• Swipe-to-dismiss gesture on bottom sheet
• Safe area insets respected on iOS
• Smooth 250ms slide animation
• Focus trapped inside open bottom sheet

Technical Notes

• Reference docs/mobile-claim-bottom-sheet.md for spec
• Use CSS custom properties for safe-area-inset-bottom
• Touch event handling for swipe gesture