feat: sender filter for message search with from:@user autocomplete

Type 'from:@name' in the search box to filter by sender — a dropdown
of matching users (avatar + display name + full ID) appears as you type
and selecting one converts it into a removable sender chip in the filter
bar. Multiple senders supported. Also works via manual entry on submit.

- SearchInput: detects trailing 'from:@...' pattern on every keystroke,
  shows PopOut autocomplete from mx.getUsers(), onMouseDown prevents
  input blur when selecting, cleans up fragment after selection
- SearchFilters: selectedSenders/onSelectedSendersChange props, sender
  chips rendered with user icon and X to remove
- useLocalMessageSearch: filters cached events by sender set when senders
  param is provided (encrypted room search respects the filter too)
- MessageSearch: handleSenderAdd deduplicates and writes to ?senders= URL
  param; localResult now passes senders to the local search

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

src/app/features/message-search/MessageSearch.tsx

@@ -214,9 +214,19 @@ export function MessageSearch({
   // eslint-disable-next-line react-hooks/exhaustive-deps
   const localResult = useMemo(() => {
     if (!msgSearchParams.term) return null;
-    return searchLocalMessages(localSearchRooms, msgSearchParams.term);
+    return searchLocalMessages({
+      term: msgSearchParams.term,
+      roomIds: localSearchRooms,
+      senders: msgSearchParams.senders,
+    });
     // eslint-disable-next-line react-hooks/exhaustive-deps
-  }, [searchLocalMessages, localSearchRooms, msgSearchParams.term, cacheVersion]);
+  }, [
+    searchLocalMessages,
+    localSearchRooms,
+    msgSearchParams.term,
+    msgSearchParams.senders,
+    cacheVersion,
+  ]);
 
   const { status, data, error, fetchNextPage, hasNextPage, isFetchingNextPage } = useInfiniteQuery({
     enabled: !!msgSearchParams.term,
@@ -297,6 +307,29 @@ export function MessageSearch({
     });
   };
 
+  const handleSelectedSendersChange = useCallback(
+    (newSenders?: string[]) => {
+      setSearchParams((prevParams) => {
+        const p = new URLSearchParams(prevParams);
+        p.delete('senders');
+        if (newSenders && newSenders.length > 0) {
+          p.append('senders', encodeSearchParamValueArray(newSenders));
+        }
+        return p;
+      });
+    },
+    [setSearchParams],
+  );
+
+  const handleSenderAdd = useCallback(
+    (userId: string) => {
+      const current = searchParamsSenders ?? [];
+      if (current.includes(userId)) return;
+      handleSelectedSendersChange([...current, userId]);
+    },
+    [searchParamsSenders, handleSelectedSendersChange],
+  );
+
   const lastVItem = vItems[vItems.length - 1];
   const lastVItemIndex: number | undefined = lastVItem?.index;
   const lastGroupIndex = groups.length - 1;
@@ -332,6 +365,7 @@ export function MessageSearch({
           searchInputRef={searchInputRef}
           onSearch={handleSearch}
           onReset={handleSearchClear}
+          onSenderAdd={handleSenderAdd}
         />
         <SearchFilters
           defaultRoomsFilterName={defaultRoomsFilterName}
@@ -343,6 +377,8 @@ export function MessageSearch({
           onGlobalChange={handleGlobalChange}
           order={msgSearchParams.order}
           onOrderChange={handleOrderChange}
+          selectedSenders={searchParamsSenders}
+          onSelectedSendersChange={handleSelectedSendersChange}
         />
       </Box>