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/sync.py

@@ -13,6 +13,7 @@ from app.sync.runner import estimate_deep_backfill
 from app.sync.subscriptions import import_subscriptions
 from app.scheduler import running_job_ids
 from app.youtube.client import YouTubeError
+from app.refusal import refused
 
 router = APIRouter(prefix="/api/sync", tags=["sync"])
 
@@ -28,10 +29,7 @@ _sync_limiter = RateLimiter(max_events=3, window_seconds=600)
 def sync_subscriptions(user: User = Depends(require_human), db: Session = Depends(get_db)) -> dict:
     if not has_read_scope(user):
         # No YouTube read grant yet — the onboarding wizard hasn't been completed.
-        raise HTTPException(
-            status_code=403,
-            detail="Connect your YouTube account (read access) to import subscriptions.",
-        )
+        raise refused(403, "youtube_read_scope")
     if not _sync_limiter.allow(str(user.id)):
         raise HTTPException(status_code=429, detail={"code": "sync_cooldown"})
     try: