fix(i18n): hold every refusal reason by type, and refuse what the scan cannot read

Review round 1 of the refusal contract: it claimed more than it held.

- The other reason carriers get their Literal too (BlockedUrl, ClipError,
  FolderError, the playlist and sign-in refusal helpers), so the type
  checker refuses a reason passed by variable or an unknown literal, and
  the contract reads one shape.
- A literal reason in a detail dict built anywhere counts as a reason of
  its code; a code the scan cannot read, a refused() whose reason it
  cannot read, and a new reason passed by variable each fail the contract
  until listed with what holds them.
- A sign-in refusal key the backend no longer sends is caught too.
- localizeDetail's numbers (download cap in GB, upload cap in bytes) and
  the shared-sentence fallbacks are tested in both languages.
- One Literal parser in the contract; a stale comment fixed.

backend/app/downloads/urlguard.py

@@ -33,22 +33,31 @@ user-controlled destination to guard.
 
 import ipaddress
 import socket
+from typing import Literal
 from urllib.parse import urlparse
 
 # Reason codes travel to the client inside the structured 422 detail, so they are part of the API
-# vocabulary (frontend `localizeDetail`) — keep them stable.
-REASON_SCHEME = "scheme"  # not http(s)
-REASON_NO_HOST = "no_host"  # URL carries no hostname
-REASON_MALFORMED = "malformed"  # unparseable authority or port; urlparse raises late, so guard it
-REASON_UNRESOLVABLE = "unresolvable"  # DNS said nothing (honest: not the same as "blocked")
-REASON_PRIVATE = "private"  # resolves into a network the server must not fetch from
+# vocabulary — keep them stable. The UI names each one (`downloads.errors.urlBlocked.<reason>`, held by
+# the frontend's backendIds contract, which reads this Literal).
+UrlBlockedReason = Literal["scheme", "no_host", "malformed", "unresolvable", "private"]
+REASON_SCHEME: UrlBlockedReason = "scheme"  # not http(s)
+REASON_NO_HOST: UrlBlockedReason = "no_host"  # URL carries no hostname
+REASON_MALFORMED: UrlBlockedReason = (
+    "malformed"  # unparseable authority or port; urlparse raises late
+)
+REASON_UNRESOLVABLE: UrlBlockedReason = (
+    "unresolvable"  # DNS said nothing (not the same as "blocked")
+)
+REASON_PRIVATE: UrlBlockedReason = (
+    "private"  # resolves into a network the server must not fetch from
+)
 
 
 class BlockedUrl(Exception):
     """A user-supplied URL that must not be fetched server-side. ``reason`` is one of the
     REASON_* codes above."""
 
-    def __init__(self, reason: str):
+    def __init__(self, reason: UrlBlockedReason):
         super().__init__(reason)
         self.reason = reason