ux: reply null state, location error feedback, retry send, reaction keyboard nav

- Reply: distinguish loading (placeholder) from not-found (null) — show
  "Original message not available" instead of a stuck loading bar
- RoomInput: geolocation errors now surface inline (denied / timed out /
  unsupported); location button shows Spinner during fetch and is disabled
- Message menu: Retry Send + Cancel Message items appear when a message
  is in NOT_SENT or CANCELLED state, calling mx.resendEvent / cancelPendingEvent
- ReactionViewer: sidebar gains role=listbox / role=option and ArrowUp/Down
  keyboard navigation between reactions

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/components/message/Reply.tsx

@@ -118,11 +118,7 @@ export const Reply = as<'div', ReplyProps>(
           data-event-id={replyEventId}
           onClick={onClick}
         >
-          {replyEvent !== undefined ? (
-            <Text size="T300" truncate>
-              {badEncryption ? <MessageBadEncryptedContent /> : bodyJSX}
-            </Text>
-          ) : (
+          {replyEvent === undefined ? (
             <LinePlaceholder
               style={{
                 backgroundColor: color.SurfaceVariant.ContainerActive,
@@ -130,6 +126,14 @@ export const Reply = as<'div', ReplyProps>(
                 maxWidth: '100%',
               }}
             />
+          ) : replyEvent === null ? (
+            <Text size="T300" truncate style={{ opacity: 0.5 }}>
+              <i>Original message not available</i>
+            </Text>
+          ) : (
+            <Text size="T300" truncate>
+              {badEncryption ? <MessageBadEncryptedContent /> : bodyJSX}
+            </Text>
           )}
         </ReplyLayout>
       </Box>