[liziye627-design]

fix(config): honor XDG_CONFIG_HOME on all platforms

Closes #176

What

config_path() used dirs::config_dir() only. On Linux that already follows XDG_CONFIG_HOME, but on macOS it always returns ~/Library/Application Support, so XDG_CONFIG_HOME was ignored there.

Lookup order is now:

1. $XDG_CONFIG_HOME/abtop/config.toml if XDG_CONFIG_HOME is set and non-empty
2. otherwise dirs::config_dir()/abtop/config.toml (unchanged previous behaviour)

The logic is a pure helper resolve_config_path(xdg_config_home, fallback), and config_path() is still the single path used by both load_config and save_theme, so loading and saving can't diverge. The README "Configuration" section now documents the lookup order.

Note for macOS users: anyone who sets XDG_CONFIG_HOME and keeps their config in ~/Library/Application Support/abtop/ will now get the XDG location instead. That's what the issue asks for, but say so if you'd prefer a fallback that also checks the old file.

Tests

• 4 new unit tests on resolve_config_path (XDG set, XDG empty, XDG unset, no base dir). They take the env value and fallback as arguments and have no #[cfg(target_os)], so they run on Linux CI. config_path_prefers_xdg_config_home failed before the fix and passes after.
• cargo test: all pass (226 unit + 1 other).
• cargo clippy --all-targets -- -D warnings: clean.
• cargo fmt --check: no diffs in the files this PR touches. It does report diffs in src/collector/claude.rs, mcp.rs and opencode.rs, which already exist on main and are left alone.

Only unit-tested on Linux (rustc 1.95.0). There was no real macOS verification. I haven't run the binary on a Mac.

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Drafted with AI assistance; the tests above were run locally.