Feature: Use toastr for space settings feedback

[mjkatgithub]

Goal

Replace inline success/error banners (e.g. UAlert "Settings saved") in space and related settings flows with non-blocking toast notifications (toastr or equivalent), so feedback matches common chat clients and does not shift page layout.

Scope

• Add a toast/notification library (or Nuxt UI toast if sufficient) and wire it app-wide for settings actions
• Migrate useSpaceSettings feedback (feedbackMessage, feedbackTone) and space settings page alerts to toasts
• Migrate role save / permission save errors currently shown via UAlert on the space settings page
• Keep error messages actionable (Matrix API errors, permission denials)
• Document usage pattern for future settings screens

Branch

feature/settings-toastr-feedback

Acceptance Criteria

• Successful saves show a short-lived toast, not a full-width banner
• Failed saves show an error toast with the server/message text
• No duplicate toast + banner for the same action
• Works in light and dark theme
• German and English strings use existing i18n keys where possible

Test Checklist

Unit

• Toast helper invoked on save success/failure (mocked)

Integration

• Manual: profile save, join rule, directory publish, role/permission save

E2E

• Optional: one smoke test for toast visibility after save

Notes

• Directory publish and join rule are separate; toasts should reflect the action that actually ran.
• Related: publish-to-directory currently used join rule; fixed separately via Matrix directory API.