feat(a11y): focus return, typing announcement, shortcuts help (P3-4)

- Focus returns to the trigger when closing 4 genuine dialogs (room-topic
  viewer, reaction viewer, header topic, Search) — 20 inline popouts/menus
  correctly left as-is (returning focus to a hover target would be wrong).
- Typing indicator announced via a visually-hidden role="status" region;
  the visual text is aria-hidden to avoid double announcement.
- New keyboard-shortcuts help dialog (press ?, ignored while typing),
  mounted in ClientNonUIFeatures.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/features/shortcuts/KeyboardShortcutsDialog.css.ts

@@ -0,0 +1,49 @@
+import { style } from '@vanilla-extract/css';
+import { DefaultReset, color, config, toRem } from 'folds';
+
+export const ShortcutList = style([
+  DefaultReset,
+  {
+    margin: 0,
+  },
+]);
+
+export const ShortcutRow = style({
+  padding: `${config.space.S100} 0`,
+});
+
+export const ShortcutTerm = style([
+  DefaultReset,
+  {
+    flexGrow: 1,
+  },
+]);
+
+export const ShortcutKeys = style([
+  DefaultReset,
+  {
+    display: 'flex',
+    alignItems: 'center',
+    gap: config.space.S100,
+    flexShrink: 0,
+  },
+]);
+
+export const Kbd = style([
+  DefaultReset,
+  {
+    display: 'inline-flex',
+    alignItems: 'center',
+    justifyContent: 'center',
+    minWidth: toRem(20),
+    padding: `0 ${config.space.S200}`,
+    height: toRem(24),
+    fontFamily: 'inherit',
+    fontSize: toRem(12),
+    lineHeight: toRem(24),
+    color: color.SurfaceVariant.OnContainer,
+    backgroundColor: color.SurfaceVariant.Container,
+    border: `${config.borderWidth.B300} solid ${color.SurfaceVariant.ContainerLine}`,
+    borderRadius: config.radii.R300,
+  },
+]);