refactor(ui): DRY the settings-card CSS tokens + fix unstyled headers (#161)

- Promote .fc-section-h to a global token (app.css). It was copied identically
  into 4 cards, and TranslationCard used the class with NO local def — so its
  section headers rendered unstyled. Now fixed everywhere.
- Promote .fc-good / .fc-weak status colours to globals; delete the local copies
  in the GPU/heads cards. (.fc-ok stays local — divergent: on-surface in
  HeadsCard vs success in QueuesTable. .fc-bad stays — different name.)
- Delete 10 identical local .fc-muted redefinitions that crept back after the
  2026-06-09 sweep; the global utility already covers them.
- DbMaintenanceCard: opacity:0.6 muted text → the .fc-muted token (the exact
  anti-pattern that token's comment forbids).
- HeadsCard: collapse byte-identical ratePct() into pct().

CSS-only + one template class swap; no logic change.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NsmJSQxnNxGgtM5Yz4GAqi

frontend/src/views/ExploreView.vue

@@ -284,7 +284,6 @@ onUnmounted(() => {
 </script>
 
 <style scoped>
-.fc-muted { color: rgb(var(--v-theme-on-surface-variant)); }
 
 /* Full-height workspace under the sticky top nav. --fc-nav-h is the nav's REAL
    measured height (set by TopNav) — a hardcoded 64px here overflowed the