fix(i18n): wrap WhatsApp token show/hide aria-label through t()

i18n audit caught one straggler — the eye-icon toggle on the access-token
input had a bare `aria-label={showToken ? 'Hide' : 'Show'}` that wouldn't
translate for screen readers on non-English locales. Switched to
`t('common.hide')` / `t('common.show')`; added the matching `common.show`
key in EN + DE (common.hide already existed).

The two remaining `placeholder=` literals in the WhatsApp tab are sample
ID strings (`123456789012345`, `gallery_ready`, `+49123456789`) — those
are identifier/value examples, not translatable English.

Other PR-touched UI surfaces passed the audit clean: 30 new i18n keys
across categories (5), settings.whatsapp (16), settings.features.whatsapp
(2), feedback (3), and the activity-log + bell entries (4) all exist in
both EN and DE.

frontend/src/features/settings/tabs/WhatsAppTab.tsx

@@ -144,7 +144,7 @@ export const WhatsAppTab: React.FC = () => {
                   type="button"
                   onClick={() => setShowToken((v) => !v)}
                   className="p-1"
-                  aria-label={showToken ? 'Hide' : 'Show'}
+                  aria-label={showToken ? t('common.hide', 'Hide') : t('common.show', 'Show')}
                 >
                   {showToken ? <EyeOff className="w-5 h-5" /> : <Eye className="w-5 h-5" />}
                 </button>