Add accessible toggle component and update layouts

[cladiousdoff-glitch]

Closes #3
Closes #4
Closes #5
Closes #6

Implemented the remaining accessibility fix.
Added a reusable semantic switch at components/ui/toggle.tsx (line 10) with:
role="switch"
aria-checked={checked}
visible On / Off text
native button click, Space, and Enter activation
Updated the Settings notification preferences in [app/(merchant)/settings/page.tsx (line 22)](C:/Users/Marvel/BettaPay-Frontend/app/(merchant)/settings/page.tsx:22) to use boolean state and the new switch component.
I also verified the other requested a11y pieces are already present:
Skip link in app/layout.tsx (line 25)
id="main-content" and tabIndex={-1} in both merchant/admin layouts
No global :focus { outline: none !important; }; only the existing :focus-visible rule remains in app/globals.css (line 77)
Verification notes: npm.cmd run lint and npx.cmd tsc --noEmit both run, but fail on pre-existing unrelated issues in dashboard/auth tests and other pages. No reported errors point to the files changed here.

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