fix(a11y,test): USelect placeholder contrast + auth logout hydration wait

a11y (main.css):
- Nuxt UI's default placeholder color (text-dimmed = #a6a09b) failed WCAG
  AA contrast on cream (2.43:1) and white (2.58:1) backgrounds, blocking
  axe checks on /member/profile (timezone) and /admin/events/create
  (tags). Override [data-slot="placeholder"] globally to var(--text-dim)
  (#5a5040), comfortably above 4.5:1 on both surfaces.

auth.spec.js (logout):
- Same hydration race as the board/admin-board-channels click tests:
  /admin's sidebar Sign-out @click handler isn't bound when Playwright
  fires the click immediately after admin-tag visibility, so the click
  no-ops and waitForResponse for /api/auth/logout times out.
- Add waitForLoadState('networkidle') after goto so hydration completes
  before the click.

app/assets/css/main.css

@@ -273,6 +273,14 @@ p a, blockquote a {
   min-width: 0;
 }
 
+/* ---- Nuxt UI placeholder contrast ----
+   Default Nuxt UI placeholder uses `text-dimmed` (#a6a09b) which fails WCAG
+   AA on cream and white backgrounds (≈2.4:1). Override globally to --text-dim
+   so USelect/USelectMenu placeholders meet the 4.5:1 ratio. */
+[data-slot="placeholder"] {
+  color: var(--text-dim);
+}
+
 /* ---- SHARED USelectMenu STYLES ----
    Apply via: <USelectMenu class="zine-select" :ui="{ content: 'tz-content', item: 'tz-item', input: 'tz-input' }" />
    Classes are global (not scoped) because Nuxt UI portals the popup content to body. */

e2e/auth.spec.js

@@ -44,6 +44,7 @@ test.describe('Authentication flows', () => {
   test('logout clears auth', async ({ page }) => {
     await loginAsAdmin(page)
     await page.goto('/admin')
+    await page.waitForLoadState('networkidle')
     await expect(page.locator('.admin-tag')).toBeVisible({ timeout: 15000 })
 
     // Set up response listener BEFORE clicking to avoid race