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/lib/components/FlagPopover.svelte

@@ -79,7 +79,7 @@
     ></textarea>
   </label>
   {#if error}
-    <p class="mt-2 text-xs text-error">Couldn't save flag — {error}</p>
+    <p class="mt-2 text-xs text-error-fg">Couldn't save flag — {error}</p>
   {/if}
   <div class="mt-3 flex justify-end gap-2">
     <button

web/src/lib/components/ToastHost.svelte

@@ -15,7 +15,7 @@
       class:border-border={toast.value.variant === 'info'}
       class:text-text-primary={toast.value.variant === 'info'}
       class:border-error={toast.value.variant === 'error'}
-      class:text-error={toast.value.variant === 'error'}
+      class:text-error-fg={toast.value.variant === 'error'}
     >
       {toast.value.message}
     </div>