Un-trap mobile nav FABs from the glass bar's…

Details

The FABs are DOM children of the .v3-nav header, whose glass
`backdrop-filter` establishes a containing block for its `position: fixed`
descendants. So the bottom-right FABs anchored to the ~57px bar at the TOP
of the screen instead of the viewport and rendered off-screen above the
fold (getBoundingClientRect top ≈ -83 / -141), invisible and unreachable
on real mobile. Drop `backdrop-filter` on .v3-nav below 700px so the FABs
anchor to the viewport; the bar's 70-78% opaque background keeps the
wordmark legible without the blur. Desktop keeps its glass.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YB1axhX3kbHCFJbX7LhfH8

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