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/schema.ts

@@ -17,6 +17,7 @@ export const users = sqliteTable('users', {
   avatarColor: text('avatar_color'),
   bio: text('bio'),
   isDeleted: integer('is_deleted').default(0),
+  discoverable: integer('discoverable').default(1),
   profileUpdatedAt: integer('profile_updated_at'),
   createdAt: integer('created_at').notNull(),
 });