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/auth.py

@@ -14,6 +14,7 @@ from app.db import get_db
 from app.models import Invite, LogEvent, User
 from app.security import decrypt_optional
 from app.utils import canonical_email
+from app.refusal import refused
 
 PASSWORD_MIN_LEN = 10
 
@@ -341,19 +342,19 @@ def current_user(request: Request, db: Session = Depends(get_db)) -> User:
 
 def _session_user(request: Request, db: Session, user_id: int | None, is_default: bool) -> User:
     if not user_id:
-        raise HTTPException(status_code=401, detail="Not authenticated")
+        raise refused(401, "not_authenticated")
     user = db.get(User, user_id)
     if user is None or not user.is_active or user.is_suspended:
         # Suspended/deactivated mid-session → end it so the block takes effect at once.
         _end_account_session(request, user_id, is_default)
-        raise HTTPException(status_code=401, detail="Not authenticated")
+        raise refused(401, "not_authenticated")
     # SA4: reject a cookie whose recorded epoch is behind the account's current one (revoked by a
     # password reset/change or "log out everywhere"). A cookie with no recorded epoch (pre-SA4, or an
     # account signed in before this shipped) is treated as epoch 0 — so the FIRST bump revokes it too.
     epochs = request.session.get("epochs") or {}
     if int(epochs.get(str(user_id), 0)) != (user.session_epoch or 0):
         _end_account_session(request, user_id, is_default)
-        raise HTTPException(status_code=401, detail="Not authenticated")
+        raise refused(401, "not_authenticated")
     # Always keep the session default in the switchable list — covers sessions created before
     # multi-session existed (their account_ids was never seeded), so the switcher isn't empty.
     default_id = request.session.get("user_id")
@@ -418,10 +419,7 @@ def require_write_scope(user: User = Depends(require_human)) -> User:
     checks that had drifted into three different 403 messages across channels/playlists. Layers on
     `require_human` because writing to YouTube needs a real, non-demo identity."""
     if not has_write_scope(user):
-        raise HTTPException(
-            status_code=403,
-            detail="Enable YouTube editing in Settings first.",
-        )
+        raise refused(403, "youtube_editing_off")
     return user
 
 
@@ -429,7 +427,7 @@ def admin_user(user: User = Depends(current_user)) -> User:
     """Dependency: require the signed-in user to be an admin. Single source of truth for the
     admin gate — every admin route/action depends on this rather than re-checking the role."""
     if user.role != "admin":
-        raise HTTPException(status_code=403, detail="Admin only")
+        raise refused(403, "admin_only")
     return user