[Bug]: [BUG] Spectacle "Save As" dialog defaults to AVCI/HEIF format, PNG/JPEG save fails — kimageformats packaging issue

### Kernel

7.2.2-1-cachyos

### Desktop Environment / Window Manager

KDE Plasma

### CPU

Intel(R) Core(TM) i7-10610U (8) @ 4.90 GHz

### GPU / Driver

Intel UHD Graphics @ 1.15 GHz [Integrated]

### Description

Intel UHD Graphics @ 1.15 GHz [Integrated]After a recent system update, Spectacle's "Save As" dialog defaults to `.avci` file extension instead of PNG. When manually selecting PNG or JPEG from the dropdown, save fails with two consecutive errors: 1. QImageWriter cannot write image: Unsupported image format 2. Cannot save screenshot. Error while writing file. The .avci file does save successfully if selected, but Dolphin cannot open it, making the format useless for daily use. IMPORTANT: This bug does NOT occur on openSUSE Tumbleweed with the same Plasma 6.7.4 + KDE Frameworks 6.29.0, confirming this is a CachyOS packaging issue, not upstream KDE.

### Steps to Reproduce

1. Press Print Screen key
2. In Spectacle, click "Save As"
3. Observe filename defaults to Screenshot_YYYYMMDD_HHMMSS.avci
4. Change file type dropdown to "PNG image"
5. Click Save
6. Error dialog appears

### Expected Behavior

"Save As" dialog should default to .png when defaultSaveImageFormat=PNG is set in ~/.config/spectaclerc. Selecting PNG from dropdown should save successfully without QImageWriter errors.

### Actual Behavior

Dialog always defaults to .avci extension. QImageWriter fails for all non-HEIF formats. The bug also affects Gwenview's "Save As" dialog, confirming this is a kimageformats / libheif plugin integration issue, not just Spectacle.

### Logs / Error Messages

Bug report log: https://paste.cachyos.org/p/9a7c5f0.log Console output shows: kf.kio.filewidgets.kfilefiltercombo: KFileFilterCombo::setCurrentFilter: Could not find file filter The [ImageSave] section in spectaclerc caches lastImageSaveAsLocation with .avci extension, which seems to override the [Save] section settings.

### Additional Context

What was tried (did NOT fix the issue):
- Set defaultSaveImageFormat=PNG in ~/.config/spectaclerc
- Cleared ~/.cache/spectacle*, ~/.cache/mime*, ~/.cache/thumbnails*
- Deleted and regenerated ~/.config/spectaclerc multiple times
- Cleared ~/.config/kfiledialog* and ~/.local/share/kfiledialog*
- Removed libheif package entirely from system
- Renamed/moved kimg_heif.so and kimg_avif.so plugins from /usr/lib/qt6/plugins/imageformats/
- Reinstalled kimageformats package
- Rebooted system multiple times
- Tested with a fresh user account — same issue persists

Current workaround: Uninstalled Spectacle, using Flameshot as replacement.

Start with the linked bug report log and the [ImageSave] and [Save] sections of ~/.config/spectaclerc, then inspect the Qt6 imageformats directory paths mentioned in the issue. Compare the CachyOS kimageformats/libheif packaging with the working openSUSE setup. Done means PNG and JPEG save successfully in Spectacle and Gwenview, with PNG restored as the default format.

Written by the indexing model from the issue text.