IssueReportPage: unmount guard on the repos fetch, aria-sort on sortable headers

Two non-blocking notes from review:
- the /api/repos effect now uses the same cancelled guard the
  repo-filter effect already has, so an unmount mid-flight doesn't call
  setRepos/setError on a gone component.
- Table's TableColumn gains an optional ariaSort field, consumed as the
  <th>'s aria-sort attribute; the issue-report page's sortable columns
  now report ascending/descending/none so a screen reader can announce
  which column and direction is active, not just the sighted ▲/▼ glyph.

frontend/packages/swarm-ui/src/ui/table/Table.tsx

@@ -20,6 +20,17 @@ export interface TableColumn<T> {
   // can pass real markup instead of forking a second table primitive.
   header: ComponentChildren;
   render: (row: T) => ComponentChildren;
+  /**
+   * ARIA sort state for this column's `<th>` — `'ascending'` /
+   * `'descending'` while this is the active sort column, `'none'` while
+   * sortable but not active, omitted entirely for a non-sortable column
+   * (no `aria-sort` attribute at all, the correct value for a column
+   * that can never be the active sort). A screen reader announces which
+   * column/direction is active from this attribute; the ▲/▼ glyph a
+   * sortable header renders is `aria-hidden` and carries no information
+   * on its own.
+   */
+  ariaSort?: 'ascending' | 'descending' | 'none';
 }
 
 export function Table<T>({
@@ -44,7 +55,9 @@ export function Table<T>({
         <thead>
           <tr>
             {columns.map((c) => (
-              <th key={c.key}>{c.header}</th>
+              <th key={c.key} aria-sort={c.ariaSort}>
+                {c.header}
+              </th>
             ))}
           </tr>
         </thead>