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/watch/watchStrings.ts

@@ -31,6 +31,7 @@ const STR = {
     open: "Open",
     folderEmpty: "This folder is empty.",
     play: "Play",
+    gifTip: "To share it in a chat, drag the downloaded file in: it moves everywhere.",
     files: (n: number) => (n === 1 ? "1 file" : `${n} files`),
   },
   hu: {
@@ -61,6 +62,7 @@ const STR = {
     open: "Megnyitás",
     folderEmpty: "Ez a mappa üres.",
     play: "Lejátszás",
+    gifTip: "Csetbe a letöltött fájlt húzd be: az mindenhol mozog.",
     files: (n: number) => `${n} fájl`,
   },
   de: {
@@ -91,6 +93,8 @@ const STR = {
     open: "Öffnen",
     folderEmpty: "Dieser Ordner ist leer.",
     play: "Abspielen",
+    gifTip:
+      "Zum Teilen in einem Chat die heruntergeladene Datei hineinziehen: Sie bewegt sich überall.",
     files: (n: number) => (n === 1 ? "1 Datei" : `${n} Dateien`),
   },
 } as const;