V3 dock: center control-pill expansion on the…

Details

The bottom control dock's toggles (location/gravity/weather/brightness)
expand into a labelled pill on hover/focus. Each button was anchored
`right: 0` inside its fixed-width slot, so the pill could only grow
leftward. Anchor it by its centre instead (left: 50% + translateX(-50%))
so it grows symmetrically and stays centred on the collapsed button's
spot. The icon stays on the right end-cap, so the off-state slash
(gravity/weather) keeps tracking it at every pill width.
Validated headless across theme (dark/light) x viewport (desktop/mobile)
x state (collapsed + all four controls hovered): every pill straddles its
slot centre symmetrically, collapsed geometry is unchanged, no neighbour
collision. Updated the now-stale comments in _toggles.scss and _dock.scss.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S2zE1oUnd7vE9VYymfS2Ab

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