feat(downloads): make a GIF in the clip panel and the editor, and show a shared one as itself

The clip panel and the editor get a Video / GIF choice with the GIF's size, the size it is expected
to come out at, and what that size means (no moving link preview over 15 MB, refused over 30 MB).
A range longer than a GIF may be leaves Make unavailable with its reason. The editor makes one GIF
per kept segment; Join says why it is unavailable instead of going dead. The share page shows a
picture as itself with its Download, and a picture in a folder opens above the list.

frontend/src/lib/api/downloads.ts

@@ -62,6 +62,8 @@ export interface ClipRequest {
   audio_track?: number;
   /** A crop frame: fractions of the box it was drawn in, and that box's shape (downloads/clip). */
   crop?: { x: number; y: number; w: number; h: number; aspect: number };
+  /** Make it a GIF, this long side in pixels (`lib/gif` GIF_SIDES); left out, an mp4. */
+  gif_side?: number;
   display_name?: string;
 }
 
@@ -74,6 +76,9 @@ export interface EditSpec {
   // scales them to the file's own pixels (a video's stored and shown sizes can differ).
   crop?: { x: number; y: number; w: number; h: number; frame_w?: number; frame_h?: number };
   accurate?: boolean;
+  /** Make it a GIF (a single trim with an end, `lib/gif` GIF_MAX_S long at most): a GIF is always
+   *  re-encoded, so `accurate` does not apply. */
+  gif?: { side: number };
 }
 
 export interface StoryboardMeta {