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The token check now reports zero superseded literals, down from 30 files, and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/assets/theme.css

@@ -132,9 +132,10 @@
   --fs-table-stripe: color-mix(in srgb, var(--fs-text-primary) 3%, transparent);
 
   /* text */
-  --fs-text-primary: #E8E4D8;  /* also the text colour on filled action buttons */
+  --fs-text-primary: #E8E4D8;  /* body, headings, labels — inverts by mode */
   --fs-text-secondary: #C2BFB4;
   --fs-text-tertiary: #9C9A92;
+  --fs-text-on-action: #E8E4D8;  /* text on a filled colour — NOT mode-dependent */
 
   /* type */
   --fs-size-display: 40px;
@@ -179,9 +180,9 @@
 }
 
 /* SUPERSEDES — write the token, not the literal.
- * #fff -> --fs-text-primary
- * #ffffff -> --fs-text-primary
- * white -> --fs-text-primary
+ * #fff -> --fs-text-on-action
+ * #ffffff -> --fs-text-on-action
+ * white -> --fs-text-on-action
  * bold -> --fs-weight-medium
  * bolder -> --fs-weight-medium
  */
@@ -290,7 +291,10 @@
      visual shifts exactly where a fallback had drifted; that shift IS the fix.
      ------------------------------------------------------------------ */
   --color-accent: var(--fs-accent);
-  --color-accent-fg: var(--fs-text-primary);
+  /* Foreground ON the accent, so it follows the accent's mode-independence,
+     not the page text's. Pointing this at --fs-text-primary made it invert to
+     obsidian on light — over a mid-tone accent, well under the AA floor. */
+  --color-accent-fg: var(--fs-text-on-action);
   --color-hover: var(--fs-surface-hover);
   --color-bg-hover: var(--fs-surface-hover);
   --color-bg-tertiary: var(--fs-surface-hover);