refactor(ui): single global .fc-muted token (DRY pattern sweep)

The muted-text token was redefined identically in 12 component <style scoped>
blocks. Consolidate to one global utility in styles/app.css; remove the 12
copies. Keeps the explicit on-surface-variant (vellum) token, NOT Vuetify's
opacity-based text-medium-emphasis (per the muted-text-token rule). Behavior-
preserving: every class=fc-muted usage now resolves to the single source.

§8b exhaustiveness caught (and I fixed) my own sed clobbering the new app.css
rule — now exactly one .fc-muted definition exists, zero component-local.
Catalog updated.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/components/settings/SystemActivityTab.vue

@@ -305,5 +305,4 @@ function formatRelative(iso) {
   cursor: pointer;
 }
 .fc-err-link:hover { text-decoration: underline; }
-.fc-muted { color: rgb(var(--v-theme-on-surface-variant)); }
 </style>