feat: add subscription search/sort, rewrite README, upgrade infrastructure

- Subscription list: search by name (case-insensitive), sort by latest
  update / name A-Z / name Z-A / unread count, preference persisted
- README: add build instructions, changelog, reorganize sections
- Upgrade Flutter SDK, Gradle/AGP/Kotlin, migrate to Dart null safety
- Bundle built-in font assets (OpenSans/Roboto/SourceSerif)
- Rename Android package to icu.cytrogen.fluent_reader
- Add i18n strings for sort/search across all 9 locales