Adds a subtle radial glow at the bottom of the viewport that emanates
from the floating search bar, fades outward toward both window corners,
and shrinks vertically as it moves away from the bar. Makes the bar
easier to spot at a glance without a heavy full-width bar or a chrome
strip.
- New `.gsearch-aura` fixed element, 260px tall, full width, pointer
events off. Radial-gradient with the accent color centered at the
bottom middle; colour stops taper 620x230px by default, ramping to
820x280px and brighter when the bar is focused/active.
- `_gsUpdateVisibility` hides the aura on /search alongside the bar
via a simple `.hidden` class.
- Focus handler adds `.active` to the aura in step with the bar;
`_gsDeactivate` removes it. z-index 99990 (below the bar at 99998,
above most page content).
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