feat: channel categories with drag-and-drop reordering

Add channel categories (named groups) with full CRUD, collapsible headers
with unread indicators, and native HTML5 drag-and-drop for reordering
channels within/between categories and reordering categories themselves.

- Schema: channel_categories table, category_id column on channels
- Server: category CRUD endpoints, batch channel-layout reorder endpoint
- WebSocket: categories in ready payload, category_created/updated/deleted
  and channel_layout_updated events with per-user VIEW_CHANNEL filtering
- Frontend: dynamic category-based sidebar layout replacing hardcoded
  Text/Voice sections, collapse state persisted to localStorage,
  category selector in CreateChannel modal, federation-aware store

packages/server/src/db/migrate.ts

@@ -111,6 +111,12 @@ export function runMigrations(db: Database.Database): void {
       columns: [
         { name: 'avatar_color', type: 'TEXT' },
       ]
+    },
+    {
+      name: 'channels',
+      columns: [
+        { name: 'category_id', type: 'TEXT' },
+      ]
     }
   ];
 
@@ -168,6 +174,17 @@ export function runMigrations(db: Database.Database): void {
     );
   `);
 
+  // Ensure channel_categories table exists (idempotent)
+  db.exec(`
+    CREATE TABLE IF NOT EXISTS channel_categories (
+      id TEXT PRIMARY KEY,
+      space_id TEXT NOT NULL REFERENCES spaces(id) ON DELETE CASCADE,
+      name TEXT NOT NULL,
+      position INTEGER DEFAULT 0,
+      created_at INTEGER NOT NULL
+    );
+  `);
+
   // Ensure voice_restrictions table exists (idempotent)
   db.exec(`
     CREATE TABLE IF NOT EXISTS voice_restrictions (