feat(downloads): a Facebook group post downloads its own video, or says it has none

Given a group post's link, yt-dlp either found no formats or took another
video from the page's feed data, so a "successful" retry of such a post was a
wrong download. With the user's cookie the Facebook adapter now reads the post
page, takes the single video node in the script blocks that carry the post id,
and downloads that video's watch link (the Chrome retry does the rest). No
single video in the post fails with its own message and the group-post copy,
reworded to say the post has no video; the post link is never handed to yt-dlp
as a fallback. Without a cookie nothing changes.

frontend/src/i18n/locales/en/downloads.json

@@ -299,8 +299,8 @@
             "body": "The cookies have probably expired, or you signed out in that browser. Export them again, upload them, and retry. If it still fails, the content has been removed or isn't visible to you."
           },
           "group_post_video": {
-            "short": "The video isn't reachable from the post link",
-            "body": "This is the link of a group post, not of the video. Open the video on Facebook and paste its link."
+            "short": "No video found in this post",
+            "body": "We couldn't find a single downloadable video in this group post. If it does have one, open the video on Facebook and paste its link."
           }
         }
       }

frontend/src/i18n/locales/hu/downloads.json

@@ -299,8 +299,8 @@
             "body": "A cookie valószínűleg lejárt, vagy kijelentkeztél abban a böngészőben. Exportáld újra, töltsd fel, és próbáld újra. Ha így sem megy, a tartalom törölt, vagy számodra nem látható."
           },
           "group_post_video": {
-            "short": "A videó nem érhető el a poszt linkjéről",
-            "body": "Ez a csoportposzt linkje, nem a videóé. Nyisd meg a videót a Facebookon, és annak a linkjét másold be."
+            "short": "Ebben a posztban nem található videó",
+            "body": "Nem találtunk egyértelműen letölthető videót ebben a csoportposztban. Ha mégis van benne, nyisd meg a videót a Facebookon, és annak a linkjét másold be."
           }
         }
       }