fix(push): push renamed branch to its same-named upstream

[patrick-cst1]

Summary

• git branch -m old new keeps the upstream config pointing at the old remote branch, so a bare git push fails with "The upstream branch of your current branch does not match".
• pushCurrentBranch now detects the name mismatch and pushes refs/heads/<newName> with -u, publishing the renamed branch and retargeting the upstream; matching-name pushes are unchanged.

Related Issue

Closes #97

Changes

• GitService.pushCurrentBranch + MainPanel routing for bare push messages.
• Tests for the renamed-upstream path and the default path.

Type of Change

• Bug fix
• Test addition or update

Testing

• New tests fail on unmodified main and pass with the fix.
• Real git check: bare git push after a rename reproduces the reported error; git push -u origin <newname> succeeds and later bare pushes work.- npx tsc --noEmit clean.
• Full suite on Windows: 260 pre-existing failures (POSIX-only integration helper + env), 0 new failures vs main; focused tests pass.
• The repo's integration suite only runs on Linux (its helper uses POSIX quoting), CI covers those.

Caveats

• The push dialog may still display the stale upstream name until the first fixed push retargets it.