feat: add pragmatic git sync — push project to external git remote

Adds a "Git Sync" section in the Integrations rail panel that lets
project owners configure an HTTPS remote URL (with embedded auth token)
and force-push all project files as a single commit.

Backend:
- GitSyncHandler: assembles project docs + binary files into a temp dir,
  runs git init/commit/push --force, then cleans up
- GitSyncController: GET/POST /project/:id/git-sync (configure),
  POST /project/:id/git-sync/push (trigger)
- Project model: gitRemote field
- Dockerfile: ensures git is present at runtime
- Env flag: OVERLEAF_ENABLE_GIT_SYNC=true (set in k8s manifest)

Frontend:
- GitSyncWidget: URL input + Save + Push Now buttons, success/error feedback
- Integrations panel: shows widget when gitSyncEnabled
- Rail: shows Integrations tab when gitSyncEnabled (was only gitBridgeEnabled)
- i18n: en + fr translations

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server-ce/config/settings.js

@@ -58,6 +58,8 @@ const settings = {
   allowAnonymousReadAndWriteSharing:
     process.env.OVERLEAF_ALLOW_ANONYMOUS_READ_AND_WRITE_SHARING === 'true',
 
+  enableGitSync: process.env.OVERLEAF_ENABLE_GIT_SYNC === 'true',
+
   // Databases
   // ---------