feat: discover people tab, privacy settings, and friend request button fix

- Add "Discover People" section to Add Friend tab with user cards, mutual counts, and inline actions
- Add discoverStore for fetching/searching discoverable users across local and federated instances
- Add PrivacyPanel to user settings with discoverability toggle
- Add is_discoverable column to users table with migration
- Fix "Send Friend Request" button vertical alignment using transform centering

packages/server/src/db/migrate.ts

@@ -132,6 +132,12 @@ export function runMigrations(db: Database.Database): void {
       columns: [
         { name: 'profile_updated_at', type: 'INTEGER' },
       ]
+    },
+    {
+      name: 'users',
+      columns: [
+        { name: 'discoverable', type: 'INTEGER DEFAULT 1' },
+      ]
     }
   ];