Add prefs, recent files, subimage/mip support

Persistent preferences and recent-file tracking plus subimage/mip handling and save support.

- Add k_imiv_prefs_filename and k_max_recent_images constants and include <cstddef>.
- Implement parsing helpers (trim, parse_bool/int/float) and clamp_placeholder_ui_state to validate UI prefs.
- Load/save persistent state to imiv_prefs.ini, including many UI options and recent_image entries; save on exit and load at startup.
- Track recent_images (normalized, deduped, capped at 16) and expose menu to open/clear recent files.
- Extend image loading to accept requested subimage and miplevel, store subimage/mip counts in LoadedImage, and adjust load_viewer_image signature and calls accordingly.
- Add UI and menu actions to change subimage and miplevel, display current/total counts, and use sensible dialog defaults (parent dir / default filename).
- Implement saving of LoadedImage buffers to disk (save_loaded_image) and integrate into Save As flow.
- Misc: default palette handling, clamp UI values in controls, and add /build_* to .gitignore.

This commit wires preferences and recent-file UX into the viewer and adds multi-subimage/mipmap support and basic image save functionality.

Author: ssh4net

.gitignore

@@ -29,3 +29,4 @@ gastest.o
 /*.log
 
 
+/build_*