fix: remove frontend auto-fill of 'main' branch in git operations

The frontend hook was defaulting to 'main' when the branch field was
empty, preventing users from leaving it blank. This complements the
backend fix in commit 558f82c by ensuring the frontend also passes
empty branch values through to the backend.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <[email protected]>

Author: sallyom

components/frontend/src/app/projects/[name]/sessions/[sessionName]/hooks/use-git-operations.ts

@@ -37,7 +37,7 @@ export function useGitOperations({
         sessionName,
         path: directoryPath,
         remoteUrl: remoteUrl.trim(),
-        branch: branch.trim() || "main",
+        branch: branch.trim(),
       });
 
       successToast("Remote configured successfully");