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/TimePicker.tsx

@@ -71,7 +71,7 @@ export const TimePicker = forwardRef<HTMLDivElement, TimePickerProps>(
                     variant={hour === selectedHour ? 'Primary' : 'Background'}
                     fill="None"
                     radii="300"
-                    aria-selected={hour === selectedHour}
+                    aria-pressed={hour === selectedHour}
                     onClick={() => handleHour(hour)}
                     disabled={(minDay && hour < minHour24) || (maxDay && hour > maxHour24)}
                   >
@@ -90,7 +90,7 @@ export const TimePicker = forwardRef<HTMLDivElement, TimePickerProps>(
                       variant={hour === selectedHour ? 'Primary' : 'Background'}
                       fill="None"
                       radii="300"
-                      aria-selected={hour === selectedHour}
+                      aria-pressed={hour === selectedHour}
                       onClick={() => handleHour(hour)}
                       disabled={
                         (minDay && hour12to24(hour, selectedPM) < minHour24) ||
@@ -109,7 +109,7 @@ export const TimePicker = forwardRef<HTMLDivElement, TimePickerProps>(
                 variant={minute === selectedMinute ? 'Primary' : 'Background'}
                 fill="None"
                 radii="300"
-                aria-selected={minute === selectedMinute}
+                aria-pressed={minute === selectedMinute}
                 onClick={() => handleMinute(minute)}
                 disabled={
                   (minDay && hour24 === minHour24 && minute < minMinute) ||
@@ -127,7 +127,7 @@ export const TimePicker = forwardRef<HTMLDivElement, TimePickerProps>(
                 variant={!selectedPM ? 'Primary' : 'SurfaceVariant'}
                 fill="None"
                 radii="300"
-                aria-selected={!selectedPM}
+                aria-pressed={!selectedPM}
                 onClick={() => handlePeriod(false)}
                 disabled={minDay && minPM}
               >
@@ -138,7 +138,7 @@ export const TimePicker = forwardRef<HTMLDivElement, TimePickerProps>(
                 variant={selectedPM ? 'Primary' : 'SurfaceVariant'}
                 fill="None"
                 radii="300"
-                aria-selected={selectedPM}
+                aria-pressed={selectedPM}
                 onClick={() => handlePeriod(true)}
                 disabled={maxDay && !maxPM}
               >