fix(composer): stop the intermittent autocomplete-insert crash

Picking an autocomplete item (mention/emoji/command — all inline voids)
occasionally tripped the composer error boundary, forcing a page refresh, even
though the element had already inserted. Root cause (traced through slate-react):
moveCursor deferred its cursor work to setTimeout(0), leaving the caret on the
just-inserted void's zero-width edge whose DOM (a U+FEFF node) isn't populated on
that tick. slate-react's commit-phase selection sync then calls
setBaseAndExtent(voidEdge, 1) and throws IndexSizeError mid-render → boundary.

Prevention: do the cursor work SYNCHRONOUSLY, in the same commit as the insert —
Transforms.move (escapes the void into the real trailing text node) then
insertText(' '). The caret is then always a resolvable text point when the
selection sync runs. (moveCursor's focus stays deferred+guarded, unchanged.)

Recovery (belt-and-suspenders): the composer error boundary is now recoverable —
a "Reload composer" button (resetErrorBoundary) + onReset Transforms.deselect
clears a transient bad selection so it remounts with the draft intact, no page
refresh. + role="alert" for screen readers.

Three review agents: two root-caused the exact slate-react throw and proved the
try/catch-only version merely recovered; a third reproduced the transforms
headlessly and caught that a first "sync insertText WITHOUT move" attempt hit
Slate's void guard (space dropped, caret trapped) — the move is required to
escape the void. Not unit-testable (needs the live DOM + the timing race).
Gate-green (tsc, eslint, prettier, 925 tests, build).

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

src/app/features/room/RoomView.tsx

@@ -1,8 +1,9 @@
 import React, { useCallback, useMemo, useRef } from 'react';
 import { ErrorBoundary } from 'react-error-boundary';
-import { Box, Text, config } from 'folds';
+import { Box, Button, Text, config } from 'folds';
 import { EventType } from 'matrix-js-sdk';
 import { ReactEditor } from 'slate-react';
+import { Transforms } from 'slate';
 import { isKeyHotkey } from 'is-hotkey';
 import { useStateEvent } from '../../hooks/useStateEvent';
 import { StateEvent } from '../../../types/matrix/room';
@@ -152,17 +153,38 @@ export function RoomView({ eventId }: { eventId?: string }) {
             <>
               {canMessage && (
                 <ErrorBoundary
-                  fallback={
+                  onReset={() => {
+                    // The composer crash is a transient bad-selection render
+                    // (e.g. after an autocomplete insert); the draft content is
+                    // intact. Clear the selection so the remounted composer can
+                    // render — the user clicks in to continue, no page refresh.
+                    try {
+                      Transforms.deselect(editor);
+                    } catch {
+                      /* editor already in a safe state */
+                    }
+                  }}
+                  fallbackRender={({ resetErrorBoundary }) => (
                     <RoomInputPlaceholder
+                      role="alert"
                       style={{ padding: config.space.S200 }}
+                      direction="Column"
                       alignItems="Center"
                       justifyContent="Center"
+                      gap="200"
                     >
-                      <Text align="Center">
-                        Message composer encountered an error. Try refreshing.
-                      </Text>
+                      <Text align="Center">The message composer hit a snag.</Text>
+                      <Button
+                        size="300"
+                        variant="Secondary"
+                        fill="Soft"
+                        radii="300"
+                        onClick={resetErrorBoundary}
+                      >
+                        <Text size="B300">Reload composer</Text>
+                      </Button>
                     </RoomInputPlaceholder>
-                  }
+                  )}
                 >
                   <RoomInput
                     room={room}