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/editor/Editor.tsx

@@ -117,7 +117,14 @@ export const CustomEditor = forwardRef<HTMLDivElement, CustomEditorProps>(
 
     const renderPlaceholder = useCallback(
       ({ attributes, children }: RenderPlaceholderProps) => (
-        <span {...attributes} className={css.EditorPlaceholderContainer}>
+        <span
+          {...attributes}
+          // [Gitea #278] Slate puts an inline `opacity: 0.333` on the placeholder,
+          // which beat EditorPlaceholderContainer's AA-contrast opacity (#222):
+          // measured 2.66:1. Drop it so the class decides.
+          style={{ ...attributes.style, opacity: undefined }}
+          className={css.EditorPlaceholderContainer}
+        >
           {/* Inner component to style the actual text position and appearance */}
           <Text as="span" className={css.EditorPlaceholderTextVisual} truncate>
             {children}