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/useLocalMessageSearch.ts

@@ -3,6 +3,12 @@ import { useCallback } from 'react';
 import { useMatrixClient } from '../../hooks/useMatrixClient';
 import { ResultGroup, ResultItem } from './useMessageSearch';
 
+export type LocalSearchParams = {
+  term: string;
+  roomIds: string[];
+  senders?: string[];
+};
+
 export type LocalSearchResult = {
   groups: ResultGroup[];
   /** How many rooms in scope are encrypted */
@@ -23,13 +29,14 @@ export const useLocalMessageSearch = () => {
   const mx = useMatrixClient();
 
   const search = useCallback(
-    (roomIds: string[], term: string): LocalSearchResult => {
+    ({ term, roomIds, senders }: LocalSearchParams): LocalSearchResult => {
       const trimmedTerm = term.trim();
       if (!trimmedTerm) {
         return { groups: [], encryptedRoomsCount: 0, searchedRoomsCount: 0 };
       }
 
       const termLower = trimmedTerm.toLowerCase();
+      const senderSet = senders && senders.length > 0 ? new Set(senders) : null;
       const groups: ResultGroup[] = [];
       let encryptedRoomsCount = 0;
       let searchedRoomsCount = 0;
@@ -56,6 +63,7 @@ export const useLocalMessageSearch = () => {
           if (event.getType() !== EventType.RoomMessage) continue;
           if (event.isDecryptionFailure()) continue;
           if (event.isRedacted()) continue;
+          if (senderSet && !senderSet.has(event.getSender() ?? '')) continue;
 
           // getContent() returns decrypted plaintext regardless of encryption
           const content = event.getContent();