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.

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