add label-based filtering on iOS

[austin-smith]

Problem

Shared filtering support for labels already exists in BitDream/Views/Shared/ContentView.swift:300, but the iOS UI only exposes status-based filters in BitDream/Views/iOS/iOSContentView.swift:211.

Goal

Expose label-based filtering on iOS.

Notes

• Existing shared helpers already support filtering by selected labels.
• iOS currently only exposes status filters from the bottom toolbar menu.

Acceptance Criteria

• iOS users can filter torrents by label, not just status.
• Label filtering integrates cleanly with the existing iOS filter UI.
• The implementation reuses the shared filtering model where possible instead of duplicating logic.
• Search, status filtering, sorting, and label filtering work together predictably.