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

src/app/components/Modal500.tsx

@@ -7,8 +7,10 @@ import { ScreenSize, useScreenSizeContext } from '../hooks/useScreenSize';
 type Modal500Props = {
   requestClose: () => void;
   children: ReactNode;
+  /** [Gitea #278] The dialog's accessible name (role="dialog" needs one). */
+  label: string;
 };
-export function Modal500({ requestClose, children }: Modal500Props) {
+export function Modal500({ requestClose, children, label }: Modal500Props) {
   const isMobile = useScreenSizeContext() === ScreenSize.Mobile;
   return (
     <Overlay open backdrop={<OverlayBackdrop />}>
@@ -26,6 +28,7 @@ export function Modal500({ requestClose, children }: Modal500Props) {
             variant="Background"
             role="dialog"
             aria-modal="true"
+            aria-label={label}
             // On mobile expand to fill the viewport. On desktop fall back to the
             // folds `size="500"` width (~50rem) — overriding maxWidth here would
             // squish the two-pane settings layout.