desktop: Clone action on Projects with external GitHub HTTPS remote fails silently — Desktop `git` process doesn't inherit system credential helper (macOS)

## Describe the bug
Clicking **Clone** on a Project whose announcement's `clone` tag points to a **private GitHub HTTPS URL** fails with only `Couldn't clone repository` in the UI. The same URL clones fine from a plain Terminal on the same machine, using the exact same user's credentials.

Root cause looks like the Tauri-spawned `git` process doesn't inherit the user's system git config (`credential.helper=osxkeychain`, `gh` credential integration, etc.) — a well-known macOS gotcha where GUI apps launched from Finder/Dock don't get the interactive shell's env.

## Steps to reproduce
1. Announce a Project whose `clone` tag is a **private** GitHub HTTPS URL (e.g. `https://github.com//.git`)
2. On macOS, in Buzz Desktop, click **Clone** on the announcement
3. Observe error: `Couldn't clone repository`
4. In a Terminal on the same machine: `git clone https://github.com//.git /tmp/x` — succeeds

## Expected behavior
Desktop's clone flow should either:
- Inherit the system git credential-helper chain (respect `~/.gitconfig`, `osxkeychain`, `gh`'s helper), or
- Prompt for GitHub credentials / offer to use an existing `gh` session, or
- Surface the underlying `git` stderr so users can self-diagnose (currently the UI swallows it)

## Version and platform
- Buzz Desktop: 0.5.11 (`CFBundleShortVersionString` from `/Applications/Buzz.app/Contents/Info.plist`)
- macOS: Darwin 25.5.0
- Bundle: `xyz.block.buzz.app`

## Related
- #5726 — adjacent: GitHub remotes not appearing in the Projects remote view

## Additional context
Public repos on GitHub HTTPS clone fine (no auth needed), so this only bites private repos. SSH URL (`git@github.com:...`) is a workaround if the user has an SSH key configured in `ssh-agent` — but same GUI-env issue likely applies there too on macOS unless `ssh-agent` is loaded system-wide.

Start at Buzz Desktop's Tauri clone flow and the spawned git process; compare its environment and credential behavior with the Terminal `git clone` reproduction for a private GitHub HTTPS URL. Done means the macOS desktop flow uses the configured credential path or surfaces the underlying git error or prompt instead of only showing `Couldn't clone repository`, while public cloning remains working.

Written by the indexing model from the issue text.