fix(i18n): every refusal the API answers is structured and said in the reader's language

~150 HTTPExceptions carried a hand-written English sentence as their detail,
shown verbatim under the Hungarian UI — in the blocking error dialog and on
every inline surface that printed HttpError.detail (sign-in, setup wizard,
change password, the passphrase gate, live search). Four also quoted an
upstream exception's text back to the user.

- app/refusal.py: `refused(status, reason, **params)` → {code: "refused",
  reason, …params}; localizeDetail translates errors.refused.<reason> with the
  params interpolated. "Unknown X" 404s use the existing not_found(what).
- sysconfig.set_value raises InvalidSetting (reason + bounds) instead of an
  English ValueError; the three writers answer it through setting_refused.
- The Plex connection/write/image failures log what the upstream said and
  answer a reason; the search scraper's failure is logged and answered as
  youtube/unavailable (it crashed refusal_reason before).
- HttpError.detail now only ever holds a translated reason: a string detail
  (only the framework's own can remain) is left out, so reading .detail is
  safe everywhere — including destructured, the hole the raw-detail source
  scan could not see. That scan and its five-file exception list are gone.
- Enforced: test_http_details.py fails on any HTTPException with a sentence
  for a detail; the contract test requires every refused reason and every
  not_found "what" in both languages, and no key for one that is gone.
- local/jsx-comment-text: a // inside JSX children renders as text; the rule
  is ten lines in eslint.config.js rather than eslint-plugin-react's ~1800
  lockfile lines.

backend/app/routes/plex/_shared.py

@@ -1,11 +1,11 @@
 """Shared helpers, serializers and constants for the plex route package."""
 
-from fastapi import HTTPException
 from sqlalchemy.orm import Session
 
 from app import logs
 from app.models import PlexItem
 from app.plex.lookups import by_rating_key
+from app.routes._common import not_found
 
 log = logs.get_logger(__name__)
 
@@ -13,7 +13,7 @@ log = logs.get_logger(__name__)
 def item_or_404(db: Session, rating_key: str) -> PlexItem:
     it = by_rating_key(db, PlexItem, rating_key)
     if it is None:
-        raise HTTPException(status_code=404, detail="Unknown Plex item")
+        raise not_found("plex_item")
     return it