Layer notifications above modal editor backdrop (#302749)

* Initial plan

* Raise notification z-index above modal editor backdrop (#302271)

Notifications (toasts and center) had z-index: 1000 which is below the
modal editor backdrop at z-index: 2540, making them hard to read when a
modal was open. Raised to 2545 so they appear above the modal backdrop
but below quick input (2550) and dialogs (2575).

Co-authored-by: bpasero <[email protected]>

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Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: bpasero <[email protected]>