Fix #2: modernize JACK config search (XDG support)

[struktured]

Closes #2

Summary

The JACK frontend's read_config() was the oldest of the three backends — C-string manipulation, only checked ~/.projectM/config.inp, no XDG support, and called abort() on common failure paths.

This replaces it with the same Qt-based search used in PipeWire (added in PR #14):

1. ~/.projectM/config.inp
2. \$XDG_CONFIG_HOME/projectM/config.inp (defaults to ~/.config per XDG spec)
3. Copy installed default into XDG location
4. Fall back to installed default
5. Return empty string → projectM uses built-in defaults

Changes

• src/ui-jack/qprojectM-jack.cpp — replace read_config() (-79 lines, +44 lines)
• src/ui-jack/CMakeLists.txt — remove unused RESOURCE_PREFIX define

Notes on the original issue

The 2021 issue mentions PROJECTM_DATADIR_PATH, which was a libprojectM 2.x build-time variable. In 4.x, preset paths come entirely from Preset Path = ... in config.inp. Honoring XDG means users can override the default config location via \$XDG_CONFIG_HOME, which is the modern equivalent.

Test plan

• Clean build with -DENABLE_JACK=ON
• ~/.projectM/config.inp still takes precedence if present
• Falls back to default config from install prefix when no user config
• Tested locally end-to-end (run projectM-jack with config in ~/.projectM/, with config in $XDG_CONFIG_HOME/projectM/, and with no user config)

[Copilot]

Pull request overview

Modernizes the JACK frontend’s config discovery to be XDG-aware and aligned with the PipeWire backend’s Qt-based implementation, avoiding legacy C-string handling and abort() paths.

Changes:

• Replaced read_config() in the JACK frontend with a Qt-based search order including ~/.projectM and $XDG_CONFIG_HOME + fallback copy behavior.
• Removed the unused RESOURCE_PREFIX compile definition from the JACK target.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/ui-jack/qprojectM-jack.cpp	Reimplements read_config() with XDG support and safer fallbacks (Qt filesystem APIs).
src/ui-jack/CMakeLists.txt	Drops now-unused RESOURCE_PREFIX define after config path logic change.

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[Copilot]

CONFIG_FILE is already defined in qprojectm_mainwindow.hpp (included above). Redefining it here is redundant and can trigger macro-redefinition warnings (and potentially fail builds that treat warnings as errors). Prefer removing this #define and using the header’s definition, or replace both with a single shared constant in a common header.

Suggested change

#define CONFIG_FILE "/share/projectM/config.inp"

[struktured]

@copilot review