fix(theme): text on a filled colour needs its own token — the old one inverts

76 hardcoded `color: #fff` now resolve to --fs-text-on-action, a new token
that is parchment in BOTH modes.

The design system said they should supersede to --fs-text-primary, on the
recorded reasoning that "there is no 'text on action' colour, there is just the
text colour." That is true on dark and wrong on light. --fs-text-primary
inverts to #14171A; the surfaces underneath it do not invert at all — every one
of these 76 sits on an action colour, a semantic colour, the accent, or the CTA
gradient, all of which hold a single value across modes.

Sweeping as recorded would have put obsidian text on moss green: roughly 2.4:1,
against a house style whose stated floor is WCAG AA. It would have looked
correct to me, because I checked it in the mode where it was correct.

--color-accent-fg had the same defect independently and is repointed too.

The token check now reports zero superseded literals, down from 30 files, and
raw colour literals drop 246 -> 169.

Closes #2275.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UaYUaouG9jjhATyuxCKrQs

frontend/src/components/ToastNotification.vue

@@ -36,7 +36,7 @@ const toastStore = useToastStore();
   gap: 0.5rem;
   padding: 0.75rem 1rem;
   border-radius: 6px;
-  color: #fff;
+  color: var(--fs-text-on-action);
   font-size: 0.9rem;
   box-shadow: 0 2px 8px var(--color-shadow);
   min-width: 200px;
@@ -54,7 +54,7 @@ const toastStore = useToastStore();
   padding: 0 0.15rem;
 }
 .toast-close:hover {
-  color: #fff;
+  color: var(--fs-text-on-action);
 }
 .toast--success {
   background: var(--color-toast-success);