feat: volume dropdown + toolbar-readable opacity floor

Mute icon now opens a slim popup with a custom vertical volume slider
(pointer/keyboard/wheel, spring-grow animation, connected nub) and a
mute toggle. Window opacity is floored at 0.85 in opacity.rs with a
complementary CSS fade on page-body children, so the toolbar stays
legible at any slider setting while page content still reads at the
expected transparency. Floor value lives only in Rust and is
substituted into the injection script via a new __FLOATVIEW_ALPHA_FLOOR__
placeholder.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: davidtorcivia