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/server/src/index.ts

@@ -20,6 +20,7 @@ import { settingsRoutes } from './routes/settings.js';
 import { utilRoutes } from './routes/utils.js';
 import { instanceRoutes } from './routes/instance.js';
 import { exploreRoutes } from './routes/explore.js';
+import { searchRoutes } from './routes/search.js';
 import { registerWebSocket } from './ws/handler.js';
 import path from 'path';
 import fs from 'fs';
@@ -79,6 +80,7 @@ async function main(): Promise<void> {
   await app.register(utilRoutes);
   await app.register(instanceRoutes);
   await app.register(exploreRoutes);
+  await app.register(searchRoutes);
   await app.register(registerWebSocket);
 
   app.get('/api/health', async () => {