feat(03-01): add SearchResults component with highlighting, scope toggle, and CommunicationHub integration

- Create SearchResults component with virtualized list, term highlighting via <mark>, keyboard navigation, loading/empty states
- Add search scope toggle (All Channels / This Channel) with radio group accessibility
- Integrate debounced (300ms) IPC search into CommunicationHub with stale response discard via requestId
- Update CommunicationSlice with search state (query, scope, results, loading, requestId)
- Modify FilterBar to accept scopeToggle prop and Escape key handler
- AnimatePresence cross-fade between SearchResults and MessageList views

src/renderer/components/communication/ChannelSidebar.tsx

@@ -202,9 +202,13 @@ export interface FilterState {
 export function FilterBar({
   filters,
   onChangeFilters,
+  scopeToggle,
+  onSearchKeyDown,
 }: {
   filters: FilterState
   onChangeFilters: (next: FilterState) => void
+  scopeToggle?: React.ReactNode
+  onSearchKeyDown?: (e: React.KeyboardEvent<HTMLInputElement>) => void
 }) {
   const t = useT()
   const [expanded, setExpanded] = useState(false)
@@ -218,11 +222,13 @@ export function FilterBar({
             type="text"
             value={filters.search}
             onChange={(e) => onChangeFilters({ ...filters, search: e.target.value })}
+            onKeyDown={onSearchKeyDown}
             placeholder={t("comm.searchMessages")}
             className="w-full rounded-[6px] border border-[color:var(--line)] bg-[color:var(--panel)] py-1.5 pl-8 pr-3 text-[11px] text-[color:var(--text)] placeholder:text-[color:var(--muted)] focus:outline-none focus:ring-1"
             style={{ focusRingColor: TERRACOTTA } as React.CSSProperties}
           />
         </div>
+        {scopeToggle}
         <button
           type="button"
           onClick={() => setExpanded(!expanded)}