Second theme-aware v1 (after add_modal_shell_v1). Same pill shape as
v0 with a `theme` param:
- light (default): byte-parity with v0 (dark #111827 pill + white fg)
- dark: INVERTED contrast — light pill (#F1F5F9) + dark fg (#0F172A)
- system: $color-text-primary bg + $color-surface fg (inverted swap)
Unlike surface-like v1s (modal-shell), toasts use inverted contrast by
design — a dark pill on light bg, a light pill on dark bg — so the
dark variant flips the pill rather than darkening it.
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