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.test.js

@@ -1,7 +1,7 @@
 import { describe, expect, test } from 'vitest';
 import { fileURLToPath } from 'node:url';
 import { dirname, resolve } from 'node:path';
-import { findCss, findTailwind, scan } from './check-tint-contrast.js';
+import { findCss, findRawText, findTailwind, scan } from './check-tint-contrast.js';
 
 const web = resolve(dirname(fileURLToPath(import.meta.url)), '..');
 
@@ -35,6 +35,20 @@ describe('hue-on-own-tint check (#3150)', () => {
     });
   });
 
+  describe('raw error text, on any surface', () => {
+    test('finds text-error, class:text-error and color: var(--fs-error)', () => {
+      expect(findRawText(`<p class="text-sm text-error">x</p>`)).toHaveLength(1);
+      expect(findRawText(`<div class:text-error={bad}>`)).toHaveLength(1);
+      expect(findRawText(`.msg { color: var(--fs-error); }`)[0].fix).toBe('var(--fs-error-fg)');
+    });
+
+    test('passes error-fg, and error as a border or outline', () => {
+      expect(findRawText(`<p class="text-error-fg">x</p>`)).toEqual([]);
+      expect(findRawText(`<div class="border-error" class:border-error={bad}>`)).toEqual([]);
+      expect(findRawText(`.a { border-color: var(--fs-error); outline-color: var(--fs-error); }`)).toEqual([]);
+    });
+  });
+
   describe('CSS', () => {
     const tinted = (color) =>
       `.pill { background: color-mix(in srgb, var(--fs-warning) 15%, transparent); ${color} }`;