feat: decouple window and column dimensions in config

[mvanhorn]

Summary

Stores volatile window/column geometry in a separate file under the XDG cache directory (~/.cache/fsearch/window.conf) instead of mixing it into the main settings file (~/.config/fsearch/fsearch.conf).

Why this matters (#659)

Users who version-control their dotfiles get noisy git diffs every time they resize the window or a column, because FSearch writes machine-specific geometry into the same file as substantive settings. Other apps (e.g. KeePassXC, cited in the issue) keep essential settings in ~/.config and volatile layout state in ~/.cache. The maintainer greenlit this direction ("That's a great idea. Added to the TODO list.").

Changes

• Moved the volatile geometry keys out of the settings sections into a new CACHE_WINDOW_SECTION: window_width, window_height, and the six *_column_width and five *_column_pos keys.
• restore_window_size, restore_column_config, restore_sort_order, the show_*_column booleans, sort_by, and sort_ascending stay in the main config file, since those are deliberate user choices rather than machine-specific geometry.
• Added config_build_cache_path() / config_build_cache_dir() / config_make_cache_dir() helpers that mirror the existing config-path helpers but are rooted at g_get_user_cache_dir().
• config_save() writes the geometry to a second GKeyFile under [Window] in the cache file. The main settings save is unaffected by a cache-write failure.
• config_load() reads geometry from window.conf. If that file is absent, it falls back to reading legacy geometry keys from the [Interface] section of fsearch.conf, so existing users keep their layout on first launch after upgrading (one-time migration; the next save relocates it to the cache file).
• Graceful degradation: if the cache directory is unwritable, the geometry is written back into the main config file as a fallback so it is never silently lost; the legacy-load path picks it back up.
• config_load_default() applies the geometry defaults.

Testing

Added src/tests/test_config.c (registered in src/tests/meson.build) covering:

• Round-trip: after save, fsearch.conf contains no window_width/*_column_width keys while window.conf does, and geometry values round-trip through a reload.
• Migration: a pre-existing fsearch.conf with legacy [Interface] geometry and no window.conf loads those values, and a subsequent save relocates them to the cache file.
• Cache-unwritable fallback: when the cache directory cannot be created, the main save still succeeds and geometry is preserved in fsearch.conf, then loads back correctly.

All three tests pass locally (built and run against glib-2.0).

Fixes #659

AI was used for assistance.

[cboxdoerfer]

In case other state options will be added in the future, I think it makes more sense to name the file something like state.ini. .ini also seems more suitable for a state file than .conf.

[cboxdoerfer]

On systems with GLIB >= 2.72 there's also support for g_get_user_state_dir (), which we should prefer over g_get_user_cache(). So we should try to load in the following order:

1. user state dir
2. user cache dir
3. user config dir

[cboxdoerfer]

Let's use STATE instead of CACHE throughout this PR.

[cboxdoerfer]

Here we should also prefer to use the g_get_user_state_dir() if our GLIB_VERSION supports it, and only use cache and config dirs as fallback.

[cboxdoerfer]

Thx for the PR.