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/styles/globals.css

@@ -254,3 +254,9 @@
 .animate-gradient-pulse {
   animation: gradientPulse 4s ease-in-out infinite;
 }
+
+@keyframes search-flash {
+  0% { background-color: rgba(124, 108, 246, 0.2); }
+  100% { background-color: transparent; }
+}
+.search-highlight { animation: search-flash 2s ease-out; }