fix(a11y): axe findings: dialog names, aria-pressed, labels, avatar alt, placeholder contrast (#278)

- Modal500 takes a label: Room/Space Settings ("<name> settings") and
  Settings dialogs have accessible names.
- aria-selected on plain buttons (inbox filter chips, date/time picker
  chips, verification/usage/notification menu items) -> aria-pressed; the
  Permissions power chips open a menu -> aria-haspopup + aria-expanded.
- Labels tied to the room pack name input and the Export From/To dates.
- UserAvatar defaults to alt="" (decorative next to a name / in a
  labelled button) unless a caller passes one.
- Composer placeholder: Slate's inline opacity 0.333 overrode the #222
  AA-contrast class (2.66:1); it is dropped so the class (0.75) applies.
- e2e/a11y.spec.ts also audits Room Settings → Permissions, Emojis &
  Stickers and Export, checks the dialog name, and audits the inbox.

Fixes #278

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017PCb59uY3UKqDqt7B6hzd5

e2e/a11y.spec.ts

@@ -118,8 +118,26 @@ test.describe('accessibility', () => {
     test('room settings', async ({ page }) => {
       await page.getByRole('button', { name: 'More Options' }).first().click();
       await page.getByText('Room Settings', { exact: true }).click();
-      await expect(page.getByRole('dialog').first()).toBeVisible();
+      // [Gitea #278] The dialog has an accessible name.
+      await expect(page.getByRole('dialog', { name: 'A11y Room settings' })).toBeVisible();
       await auditPage(page, 'room settings');
+      // [Gitea #278] aria-selected on the power chips, unlabelled pack name and
+      // export date inputs were only on these pages.
+      // One page at a time, in order.
+      for (const name of ['Permissions', 'Emojis & Stickers', 'Export']) {
+        // eslint-disable-next-line no-await-in-loop
+        await page.getByRole('dialog').getByText(name, { exact: true }).first().click();
+        // eslint-disable-next-line no-await-in-loop
+        await page.waitForTimeout(1000);
+        // eslint-disable-next-line no-await-in-loop
+        await auditPage(page, `room settings: ${name}`);
+      }
+    });
+
+    test('inbox invites', async ({ page }) => {
+      await page.goto('/inbox/invites/');
+      await expect(page.getByRole('button', { name: /Known|Unknown|Spam/ }).first()).toBeVisible();
+      await auditPage(page, 'inbox invites');
     });
   });
 });