Fix: append .bak extension instead of replacing file extension

[prefrontalsys]

Summary

• path.with_extension("md.bak") replaces the existing extension rather than appending
• For .md files this works by coincidence (CLAUDE.md → CLAUDE.md.bak)
• For .json files it would produce settings.md.bak instead of settings.json.bak
• Uses with_file_name(format!("{}.bak", file_name)) to properly append, matching the pattern in utils/backup.rs

Test plan

• Pull with existing global CLAUDE.md — verify backup is created as CLAUDE.md.bak
• If testing with other file types, verify extension is preserved (e.g. foo.json → foo.json.bak)

Relates to #195

🤖 Generated with Claude Code

[tylergraydev]

Good fix at line 550, but line 673 in pull_from_gist has the exact same bug:

let backup = claude_md_path.with_extension("md.bak");

This needs the same with_file_name(format!("{}.bak", file_name)) treatment. Once that's addressed, this is good to go.

[prefrontalsys]

Good catch — pushed the same fix to line 674 (now uses with_file_name(format!("{}.bak", file_name))). Also rebased onto latest main and fixed a stray rustfmt comment-alignment issue in cloud_sync.rs that surfaced after the rebase.

All Rust tests, Clippy, Format, and frontend-tests pass. The 4 build jobs fail with failed to decode secret key: incorrect updater private key password — that's expected fork-PR behavior since GitHub Actions doesn't share the Tauri signing secret with external fork PRs. Builds on main post-merge should be fine.

[tylergraydev]

Both occurrences fixed, rebase looks clean. The 4 failing build checks are expected fork-PR behavior (Tauri signing secret). LGTM.