SW-2072 — AI chat input grey by default

The InputGroup greyed the whole field via has-disabled whenever it contained any disabled descendant. The chat's submit button is disabled while the input is empty, so the field went grey even though it was available. Fix: scope the greying to the control — has-[[data-slot=input-group-control]:disabled].

Before
has-disabled:bg-input/50 — the disabled submit button greys the entire field, though the input is usable.
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Claude Sonnet ⌄  ◍ Search
After · available
Scoped to the control: a disabled submit no longer greys the field. The empty/available chat is white.
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Claude Sonnet ⌄  ◍ Search
After · unavailable
When the control itself is disabled (chat genuinely unavailable), the field still greys — as intended.
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Claude Sonnet ⌄  ◍ Search