Fix shell command substitution in binary path configuration

[BrianGreenhill]

Description

Add support for shell command substitutions (e.g., $(which aerospace)) in binary path configuration options. This allows portable configuration files that work across different architectures and installation paths.

Previously, the default settings advertised support for $(which ...) syntax in paths like aerospacePath, yabaiPath, etc., but this was never actually implemented. The literal string "$(which aerospace)" would be used as the command, causing execution failures.

This commit adds proper expansion of shell substitutions when loading the external config file:

• Expands $(which ...) patterns by setting PATH to include common Homebrew locations (/opt/homebrew/bin, /usr/local/bin) before evaluation
• Works in Übersicht's restricted environment where user PATH is not available
• Preserves original substitution syntax when saving config back to disk, so $(which aerospace) remains in the file rather than being replaced with the expanded path
• Handles expansion failures gracefully by falling back to original value

This enables truly portable configuration files that work across:

• Apple Silicon Macs (Homebrew in /opt/homebrew)
• Intel Macs (Homebrew in /usr/local)
• Different installation methods and paths

Binary path options affected:

• global.aerospacePath
• global.yabaiPath
• global.flashspacePath
• githubWidgetOptions.ghBinaryPath
• mpdWidgetOptions.mpdBinaryPath
• gpuWidgetOptions.gpuMacmonBinaryPath

Fixes # #492

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Test A: Shell substitution expansion on config load

1. Set "aerospacePath": "$(which aerospace)" in ~/.simplebarrc
2. Restart Übersicht
3. Verify in Debug Console that path expands correctly (e.g., to /opt/homebrew/bin/aerospace)
4. Verify simple-bar loads without errors
5. Verify workspace switching and window management functions work

Test B: Config preservation on save

1. Set "aerospacePath": "$(which aerospace)" in ~/.simplebarrc
2. Make a change to simple-bar settings via UI
3. Verify ~/.simplebarrc still contains "$(which aerospace)" (not the expanded path)
4. Restart Übersicht
5. Verify expansion still works after reload

Test C: Fallback behavior

1. Set "aerospacePath": "$(which nonexistent)" in ~/.simplebarrc
2. Verify simple-bar handles gracefully (uses original value, logs warning)
3. Revert to valid path and verify recovery

Test Configuration:

• OS version: macOS Tahoe 26.1
• AeroSpace version: v0.19.2-Beta (d246f250468fc9d427a2eb901d56794af7ac6609)
• Übersicht version: 1.6 (82)
• Architecture: Apple Silicon (M3)

Additional Testing Notes:

• Tested with aerospace installed via Homebrew at /opt/homebrew/bin/aerospace
• Verified workspace data fetching, window management, and display queries all function correctly
• Checked Debug Console for expansion logs showing successful path resolution

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas (make use of JSDoc if necessary)
• My changes generate no new warnings

Changes to make to the documentation

Please list any changes to the documentation that are required to reflect your changes.