Fix Edit button vertical alignment in Identity Overrides row

## Problem

In the Edit Feature modal → Identity Overrides tab, the Edit button is vertically misaligned with the other row elements (toggle, identity name, value chip).

**Location**: `web/components/modals/create-feature/tabs/IdentityOverridesTab.tsx` lines 256–262

The Edit button (text + icon) has a different layout from the adjacent delete button (icon-only), causing vertical misalignment in the row.

## Fix

Add `align-items-center` to the outer row container.

## Acceptance Criteria

- [ ] Edit button and delete icon are vertically centred with the toggle and identity name

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Part of the Design System Audit (#6606) · Epic: #6882