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The token check now reports zero superseded literals, down from 30 files, and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/ShareDialog.vue

@@ -268,7 +268,7 @@ onMounted(async () => {
 .share-tab.active {
   background: var(--color-primary);
   border-color: var(--color-primary);
-  color: #fff;
+  color: var(--fs-text-on-action);
 }
 
 .share-target-form {
@@ -339,7 +339,7 @@ onMounted(async () => {
 .btn-add-share {
   padding: 0.45rem 1rem;
   background: var(--gradient-cta);
-  color: #fff;
+  color: var(--fs-text-on-action);
   border: none;
   border-radius: 6px;
   font-size: 0.85rem;