Fix the git path fallback freezing launch when validation fails - #766

Run the default git search off the main thread and keep the configured path until a replacement is found, so a failed background check no longer blocks the UI or leaves the git path nil while the search runs.

This continues #748 for #744, which was reported again on 1.7.1. This PR alone does not fix #744 and does not close it. Manual testing found the likely root cause: a race in PBTask that sometimes returns empty output for a successful git --version when many instances start at once. That is fixed separately in #774. This PR remains useful on top of it: a configured path that fails validation no longer blocks startup, and falls back to a working git.

• Search for a default git binary on the background queue without touching the shared state
• Switch to the binary found and show the alert on the main thread
• Keep the configured path when no fallback is found
• Build the search locations list once, as it is now read off the main thread
• Log which binary replaced the configured one

Refs #744, see also #774

Test plan (run on the CI arm64 build of this branch):

• Launch normally: no alert, repo opens
• Launch with -gitExecutable pointing at a script that exits 1: the log shows the failed validation and the binary used instead (/opt/homebrew/bin/git), the alert is shown, the app stays usable
• Launch with -gitExecutable pointing at a script that sleeps 8 s before running git: the UI is responsive during the wait, and no alert appears
• Launch about a dozen repositories at once: reproduced the empty git --version output now fixed in Fix commands sometimes returning empty output when many run at once #774
• Launch with a missing path: alert shown immediately, as before