feat: implement message search and clean up header buttons

Remove placeholder buttons (Threads, Inbox, Help) from channel and DM
headers. Add full-text message search with backend endpoints for both
space channels and DMs, supporting filters (from, has, before, after)
and pagination. Search popover with debounced input, highlighted matches,
and jump-to-message that scrolls with a highlight animation. Includes
messages/around endpoints for loading context when jumping to uncached
messages.

packages/web/src/components/chat/Message.tsx

@@ -155,6 +155,7 @@ export function Message({ message, isCompact, isFirstInGroup }: MessageProps) {
 
   const content = (
     <div
+      id={`msg-${message.id}`}
       className={`group relative flex px-5 py-[3px] transition-colors ${isFirstInGroup || message.replyTo ? 'mt-[1.0625rem]' : ''} ${
         isMentioned
           ? 'bg-accent-amber/10 border-l-2 border-l-accent-amber hover:bg-accent-amber/15'