feat: support force fetch

[Jamie-Cui]

Summary

• add git-overleaf-overwrite-local as the destructive counterpart to remote overwrite
• preserve the previous committed HEAD with a safety ref, then replace the current branch, index, and worktree from the downloaded Overleaf snapshot
• route Magit fetch -f / --force through the new command while keeping ordinary fetch non-destructive
• clear pending pull metadata only after a successful local overwrite
• document the workflow in English and Chinese and add command, Git-state, async, and Magit regression coverage

Why

Ordinary git-overleaf-fetch intentionally updates only the hidden remote snapshot ref. Users also need an explicit remote-wins recovery path corresponding to force-push, especially when abandoning local divergence or a pending conflicted pull.

User impact

The new interactive command always asks for confirmation. It discards staged, unstaged, and non-ignored untracked changes, preserves ignored files, rejects detached HEAD, and keeps the previous committed HEAD reachable under refs/git-overleaf/backups/ when local backups are enabled.

Validation

• make test — 105/105 ERT tests passed
• make — byte compilation passed
• batch load smoke test for git-overleaf and git-overleaf-magit passed
• live Emacs byte compilation of the changed package files passed without project warnings
• git diff --check passed