fix(web): error text uses error-fg on every surface, not only on tints (#3150)

Raw error red fails AA as text even with no tint behind it: in dark mode
it measures 3.63:1 on obsidian, 3.23 on iron and 2.64 on slate, against
4.5. error-fg (the house formula, 50% toward parchment) measures 5.30 at
worst across both modes.

- All 19 text-error uses become text-error-fg: the "Couldn't load"
  messages on the admin pages, the integrations form errors, the flag
  popover, and the error toast's text. The toast keeps its error border,
  since a border is a graphic with a 3:1 floor.
- check-tint-contrast flags raw error text anywhere (text-error,
  class:text-error, color: var(--fs-error)) and leaves borders and
  outlines alone. Run against the files before the swap, it finds all 19.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/scripts/check-tint-contrast.js

@@ -64,6 +64,27 @@ export function findCss(source) {
   return found;
 }
 
+// Hues that fail AA as text even on a plain surface, so the raw hue is never
+// text at all: error measures 2.64:1 on slate and 3.23 on iron in dark mode,
+// against 4.5; error-fg measures 5.30 at worst. A border or outline in the
+// hue is a graphic with a 3:1 floor and stays allowed.
+export const TEXT_NEVER_RAW = ['error'];
+
+/** Raw text-X classes and color: var(--fs-X) for the hues above. */
+export function findRawText(source) {
+  const found = [];
+  for (const hue of TEXT_NEVER_RAW) {
+    const patterns = [
+      [new RegExp(`(?<=^|[\\s'"\`:])text-${hue}(?=$|[\\s'"\`=])`, 'g'), `text-${hue}-fg`],
+      [new RegExp(`(?<![-\\w])color\\s*:\\s*var\\(--fs-${hue}\\)`, 'g'), `var(--fs-${HUES[hue]})`]
+    ];
+    for (const [re, fix] of patterns) {
+      for (const m of source.matchAll(re)) found.push({ hue, index: m.index, fix });
+    }
+  }
+  return found;
+}
+
 function lineOf(source, index) {
   return source.slice(0, index).split('\n').length;
 }
@@ -81,7 +102,7 @@ export function scan(root, base = root) {
   const out = [];
   for (const path of walk(root)) {
     const source = readFileSync(path, 'utf8');
-    for (const f of [...findTailwind(source), ...findCss(source)]) {
+    for (const f of [...findTailwind(source), ...findCss(source), ...findRawText(source)]) {
       out.push(`${relative(base, path)}:${lineOf(source, f.index)} ${f.hue} → ${f.fix}`);
     }
   }