feat(clip): crop a clip to a frame drawn on the picture, and give the clip panel the room it needs

An optional crop in clip mode: a Crop toggle puts a frame on the picture (free, 16:9, 9:16 or 1:1;
the whole picture stays the default). The frame is sent in fractions of the box it was drawn in,
with that box's shape; the worker fits the real picture into the box by its display aspect (sample
aspect and rotation aware), drops the part over the bars, and crops first in the filter chain. A
frame covering the whole box is no crop at all, and one off the picture is refused by name.

The crop frame is a ui primitive (CropFrame) shared with the video editor, keyboard-operable.
YouTube: the wide side panel shuts while clipping and comes back after; maximised, the clip panel
docks in the stage. Plex: the picture sits above the clip panel so the frame lies on it.

backend/app/downloads/pipeline.py

@@ -985,6 +985,20 @@ def _clip_source(
     return src, spec["start_s"], None
 
 
+def _clip_crop(spec: dict, data: dict) -> str:
+    """The clip's crop filter, from its frame and the picture the file really holds ("" for none)."""
+    frame = spec.get("crop")
+    if not frame:
+        return ""
+    dar = clipmod.display_aspect(data)
+    if dar is None:
+        raise StatedFailure("The picture's size could not be read, so the crop cannot be placed.")
+    crop = clipmod.crop_filter(frame, dar)
+    if crop is None:
+        raise StatedFailure("The crop frame lies outside the picture.")
+    return crop
+
+
 def _process_clip(job_id: int, asset_id: int) -> None:
     """Cut an ad-hoc clip (downloads/clip): fetch or open its source, ONE re-encode to the clip
     shape, then into the user's library beside their edits."""
@@ -1004,6 +1018,7 @@ def _process_clip(job_id: int, asset_id: int) -> None:
         audio_track = spec.get("audio_track")
         if audio_track is not None and audio_track >= clipmod.audio_track_count(data):
             raise StatedFailure("The chosen audio track is not in the file.")
+        crop = _clip_crop(spec, data)
         total_s = clipmod.duration(spec)
         out = staging / "clip.mp4"
         _set_job(job_id, phase="editing", progress=0, speed_bps=None, eta_s=None)
@@ -1016,6 +1031,7 @@ def _process_clip(job_id: int, asset_id: int) -> None:
                 audio_track=audio_track,
                 hdr=clipmod.is_hdr(data),
                 threads=editmod.encode_thread_cap(),
+                crop=crop,
             ),
             total_s,
             on_progress=_ffmpeg_progress_writer(job_id, "editing"),