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

feat: enhance custom font upload with WebView rendering, WOFF/WOFF2 support, preview & deletion

- Fix core bug: custom fonts now actually render in article WebView via
  @font-face injection and Jaguar local server font serving route
- Also fix built-in fonts (OpenSans/Roboto/SourceSerif) not rendering in WebView
- Add WOFF/WOFF2 format support for font upload
- Add font preview text in font picker and selected font indicator
- Add custom font deletion UI with confirmation dialog
- Add font file size validation (1KB-50MB)
- Update CSP to allow font-src from local server
- Add missing i18n translations for all 9 locales (zh, de, es, fr, hr, pt, tr, uk)
- Update Android build configuration and dependencies

Add Croatian translation (#43)

intl_hr.arb