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/components/settings/GpuAgentCard.vue

@@ -367,11 +367,6 @@ async function onReprocess() {
 </script>
 
 <style scoped>
-.fc-muted { color: rgb(var(--v-theme-on-surface-variant)); }
-.fc-section-h {
-  font-size: 13px; font-weight: 700; letter-spacing: 0.03em;
-  text-transform: uppercase; color: rgb(var(--v-theme-on-surface));
-}
 .fc-token {
   display: flex; align-items: center; gap: 4px;
   background: rgb(var(--v-theme-surface-light)); border-radius: 6px;
@@ -390,6 +385,4 @@ async function onReprocess() {
   font-size: 11px; text-transform: uppercase; letter-spacing: 0.04em;
   color: rgb(var(--v-theme-on-surface-variant));
 }
-.fc-good { color: rgb(var(--v-theme-success)); }
-.fc-weak { color: rgb(var(--v-theme-error)); }
 </style>