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/styles/app.css

@@ -40,6 +40,20 @@
    emits, so no specificity/reorder fight — no !important needed. */
 .fc-muted { color: rgb(var(--v-theme-on-surface-variant)); }
 
+/* Section sub-heading in settings cards (DRY pass #161): was redefined
+   identically in 4 cards, and TranslationCard used the class with NO local def
+   so its section headers rendered unstyled. Now one global utility. */
+.fc-section-h {
+  font-size: 13px; font-weight: 700; letter-spacing: 0.03em;
+  text-transform: uppercase; color: rgb(var(--v-theme-on-surface));
+}
+
+/* Status text colours (DRY pass #161): fc-good = success, fc-weak = error,
+   consolidated from the GPU / heads cards. fc-ok is intentionally NOT global —
+   it means on-surface in HeadsCard but success in QueuesTable. */
+.fc-good { color: rgb(var(--v-theme-success)); }
+.fc-weak { color: rgb(var(--v-theme-error)); }
+
 /* Vuetify 4 dropped its global CSS reset (normalisation moved into each
    component). FC's layouts assumed the reset zeroed margins on text elements, so
    restore just that — the "minimal reset" from the v4 upgrade guide — inside