fix(a11y): aria-label on icon-only buttons (Subscription copy + InstallationGuide back/QR)

Subscription copy-URL button only had a title attribute; back and QR-open buttons in
InstallationGuide had no accessible name. Add aria-label so screen readers announce
the action (WCAG 2.4.6).

src/pages/Subscription.tsx

@@ -1023,6 +1023,7 @@ export default function Subscription() {
                         : `1px solid ${g.trackBg}`,
                       color: copied ? 'rgb(var(--color-accent-400))' : g.textMuted,
                     }}
+                    aria-label={t('subscription.copyLink')}
                     title={t('subscription.copyLink')}
                   >
                     {copied ? <CheckIcon /> : <CopyIcon />}