preview: remove ffprobe fallback and unused json import

_get_image_dimensions now relies solely on pyvips header reading.
If pyvips cannot read the header the function returns None and the
caller encodes at full resolution rather than falling back to ffprobe.
Remove unused stdlib json import.

cista/preview.py

@@ -2,7 +2,6 @@ import asyncio
 import contextlib
 import gc
 import io
-import json
 import mimetypes
 import struct
 import subprocess
@@ -706,37 +705,13 @@ def _get_image_dimensions(path: Path) -> tuple[int, int] | None:
     """Probe image dimensions.
 
     pyvips can read the header of most formats (including HEIC) without
-    fully decoding the image.  ffprobe is used as a fallback.
+    fully decoding the image.
     """
     try:
         img = pyvips.Image.new_from_file(str(path))
         return img.width, img.height
     except pyvips.error.Error:
-        pass
+        return None
-    try:
-        result = subprocess.run(
-            [
-                "ffprobe",
-                "-v",
-                "error",
-                "-select_streams",
-                "v:0",
-                "-show_entries",
-                "stream=width,height",
-                "-of",
-                "csv=s=x:p=0",
-                str(path),
-            ],
-            capture_output=True,
-            text=True,
-            check=True,
-        )
-        parts = result.stdout.strip().split("x")
-        if len(parts) == 2:
-            return int(parts[0]), int(parts[1])
-    except Exception:
-        pass
-    return None
 
 
 def _image_via_ffmpeg(path: Path, maxsize: int, quality: int) -> bytes: