fix(#71): announce theme toggle state

[edehvictor]

Summary

• expose the theme toggle state with �ria-pressed so screen readers can tell when dark mode is active
• keep the existing action label, but derive it from the resolved mounted theme state used by the toggle icon
• add focused TopBar coverage for the dark and light theme toggle announcements

Closes #71
Closes #67
Closes #63
Closes #56

Type of change

• Bug fix
• Feature
• Localization (i18n)
• Accessibility
• Smart contract / on-chain wiring
• Docs / chore

Screenshots / screencast

No visible UI change beyond accessibility semantics.

Checklist

• Tied to an accepted issue (Closes Announce theme toggle state #71)
• �un run build passes locally (builds + type-checks)
• Follows the design system — no layout or copy changes
• User-facing strings added to both messages/en.json and messages/fr.json (not needed)
• Accessible — the theme toggle now exposes pressed state
• No secrets committed
• Docs updated where relevant (not needed)

Testing

Not run locally.

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