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/time-date/DatePicker.tsx

@@ -69,7 +69,7 @@ export const DatePicker = forwardRef<HTMLDivElement, DatePickerProps>(
                   variant={selectedDay === day ? 'Primary' : 'SurfaceVariant'}
                   fill="None"
                   radii="300"
-                  aria-selected={selectedDay === day}
+                  aria-pressed={selectedDay === day}
                   onClick={() => handleDay(day)}
                   disabled={
                     (selectedYear === minYear && selectedMonth === minMonth && day < minDay) ||
@@ -90,7 +90,7 @@ export const DatePicker = forwardRef<HTMLDivElement, DatePickerProps>(
                   variant={selectedMonth === month ? 'Primary' : 'SurfaceVariant'}
                   fill="None"
                   radii="300"
-                  aria-selected={selectedMonth === month}
+                  aria-pressed={selectedMonth === month}
                   onClick={() => handleMonth(month)}
                   disabled={
                     (selectedYear === minYear && month < minMonth) ||
@@ -115,7 +115,7 @@ export const DatePicker = forwardRef<HTMLDivElement, DatePickerProps>(
                   variant={selectedYear === year ? 'Primary' : 'SurfaceVariant'}
                   fill="None"
                   radii="300"
-                  aria-selected={selectedYear === year}
+                  aria-pressed={selectedYear === year}
                   onClick={() => handleYear(year)}
                 >
                   <Text size="T300">{year}</Text>