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/src/routes/admin/requests/+page.svelte

@@ -198,7 +198,7 @@
   {#if query.isPending}
     <p class="text-text-secondary">Reading the queue…</p>
   {:else if query.isError}
-    <p class="text-error">Couldn't load requests.</p>
+    <p class="text-error-fg">Couldn't load requests.</p>
   {:else if rows.length === 0}
     <p class="text-text-secondary">Nothing here.</p>
   {:else}