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/playlists.py

@@ -2,7 +2,7 @@
 
 from datetime import datetime, timezone
 
-from fastapi import APIRouter, Depends, HTTPException
+from fastapi import APIRouter, Depends
 from pydantic import BaseModel
 from sqlalchemy import delete, func, select, update
 from sqlalchemy.orm import Session
@@ -19,7 +19,8 @@ from app.models import (
 )
 from app.plex.cards import leaf_card
 from app.plex.lookups import by_rating_key
-from app.routes._common import owned_or_404
+from app.routes._common import owned_or_404, not_found
+from app.refusal import refused
 
 router = APIRouter(prefix="/api/plex", tags=["plex"])
 
@@ -116,7 +117,7 @@ def create_playlist(
 ) -> dict:
     title = body.title.strip()
     if not title:
-        raise HTTPException(status_code=400, detail="title is required")
+        raise refused(400, "title_required")
     pl = PlexPlaylist(user_id=user.id, title=title)
     db.add(pl)
     db.flush()
@@ -182,7 +183,7 @@ def rename_playlist(
     pl = owned_or_404(db, PlexPlaylist, pid, user, what="playlist")
     title = body.title.strip()
     if not title:
-        raise HTTPException(status_code=400, detail="title is required")
+        raise refused(400, "title_required")
     pl.title = title
     db.commit()
     count = (
@@ -219,7 +220,7 @@ def playlist_add_item(
     pl = owned_or_404(db, PlexPlaylist, pid, user, what="playlist")
     it = by_rating_key(db, PlexItem, body.item_rating_key)
     if it is None:
-        raise HTTPException(status_code=404, detail="Unknown item")
+        raise not_found("item")
     exists = (
         db.execute(
             select(PlexPlaylistItem).where(