chore: bump version to v2026.4.309 and add v2026.4.307 changelog}
- Bump package.json version from v2026.4.308 to v2026.4.309 - Update build references from v2026.4.307 to v2026.4.308 across docs - Add detailed changelog entry for v2026.4.307 release covering: - Bootstrap 3→5, jQuery 3→4, CodeMirror 5→6 upgrades - Symfony 7.x framework migration replacing Silex - i18n support with 29 languages - Network graph, treegraph infinite scroll, blame view redesign - LESS→SCSS migration, Docker setup, CI upgrades - Various bug fixes and dependency cleanups