fix(plugins): cd into mktemp before git clone to survive dead cwd

[Mic92]

Motivation

If noctalia-shell's working directory is removed while it runs (started from a temp dir, dotfiles manager relinks the launch dir, cd into a since-deleted build tree, ...), the plugin fetch/install subprocesses inherit that dead cwd and git fails with:

shell-init: error retrieving current directory: getcwd: cannot access parent directories: No such file or directory

With stderr redirected to /dev/null this surfaced only as Empty response from <source> — no hint at the actual cause, which made it look like a network/manifest problem.

This PR:

• extracts a small sparseCloneCmd() helper shared by fetch and install that cds into the mktemp dir before running git, so the inherited cwd no longer matters;
• stops discarding stderr and includes it in the error log/callback so future failures are diagnosable.

Type of Change

• Bug fix
• New feature
• Breaking change
• Refactoring

Related Issue

• n/a

Testing

• Tested on niri
• Tested on Hyprland
• Tested on sway
• Tested with different bar positions and density settings
• Tested at different interface scaling values
• Tested with multiple monitors (if applicable)

Reproduced by launching noctalia-shell from a directory, removing that directory, then triggering a plugin fetch from the UI. Before: Empty response from <source>. After: fetch succeeds; if git itself fails, its stderr is shown in the log/toast.

Screenshots / Videos

n/a (no visual change)

Checklist

• Code follows project style guidelines
• Self-reviewed my code
• No new warnings or errors
• Documentation or comments updated (if relevant)

Additional Notes

Happy to split the stderr-surfacing into a separate PR if you'd prefer the cwd fix on its own.