fix(downloads): iPad audio playback + honest non-retryable failure for auth-walled hosts

5a — the public watch page rendered every asset in a <video>, but WebKit (all
iOS browsers) refuses to play an audio-only mp3 in <video>, so shared audio
links were broken on iPad while working on desktop Chromium. public_meta now
flags audio-only assets (codec present, no video stream) and WatchPage renders
a native <audio> in a padded surface card (no artwork exists for these).

5c — a login-walled Facebook post failed as a bare 'unknown', which offered a
misleading Retry. Classify closed-group / cookie-suggestion auth walls as
login_required and genuinely unsupported sites as a new unsupported_host class
(both non-retryable), and drop unknown from the modal's retryable set — an
unclassified failure is most often an unparsed auth wall. The queue row keeps
its own force-retry for real transients.

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

@@ -220,6 +220,10 @@
           "short": "Sign-in required",
           "body": "This video needs a signed-in, age-verified, or members-only account to view, so it can't be downloaded here."
         },
+        "unsupported_host": {
+          "short": "Site not supported",
+          "body": "This site isn't supported for downloading, so nothing could be fetched from this link."
+        },
         "geo_blocked": {
           "short": "Region-blocked",
           "body": "The source blocks this video in the server's region, so it can't be downloaded."

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

@@ -220,6 +220,10 @@
           "short": "Bejelentkezés szükséges",
           "body": "Ehhez a videóhoz bejelentkezett, kor-ellenőrzött vagy tagsági fiók kell, ezért itt nem tölthető le."
         },
+        "unsupported_host": {
+          "short": "Nem támogatott oldal",
+          "body": "Ez az oldal nem támogatott a letöltéshez, ezért erről a linkről semmit sem sikerült lekérni."
+        },
         "geo_blocked": {
           "short": "Régió szerint tiltva",
           "body": "A forrás a szerver régiójában letiltja ezt a videót, ezért nem tölthető le."