Modernize navigation with center FAB and inline barcode scanner

[bl4ckh4nd]

Summary

• Redesigned bottom navigation with a raised center FAB (+) button replacing the flat 6-item layout — Dashboard and Products on the left, Stats and Settings on the right
• Removed the standalone Scanner page from the navigation; barcode scanning is now integrated directly into the Add Product form as a collapsible inline scanner
• Duplicate detection and Open Food Facts API lookup work the same as before, now within the form itself

Test plan

• Tap the center FAB (+) — should navigate to the Add Product page
• Expand the inline scanner, scan a barcode — barcode, name, and photo should auto-fill
• Scan a duplicate barcode — duplicate popup should appear with existing product details
• Scan while offline — only barcode field should be filled
• Edit an existing product — scanner section should not appear
• All four nav items (Dashboard, Products, Stats, Settings) navigate correctly
• Dashboard quick-action buttons navigate to the Add Product page