feat(plex): field-scoped search (S2) + instant search typeahead

Add a search scope-pill to the Plex header search box (GitHub-style, one
field-scope at a time) and an instant typeahead of the top matches below it.

Backend (unified_library):
- New `field` param: all | title | people | summary | episode. `all` is
  byte-identical to the pre-scope behaviour (untouched weighted search_vector);
  a specific field keeps the GIN `search_vector @@ tsq` prefilter AND ANDs a
  per-column tsvector recheck, so scoped hits are a strict subset of Everywhere.
  Per-field expressions mirror the search_vector Computed def (same TS config,
  same left(summary,1000) truncation).
- `episode` scope empties the movie/show grid and title-only-matches the grouped
  Episodes section; the title/people/summary scopes suppress that section.
- Unknown field falls back to `all`. New DB-lane characterization tests
  (test_plex_field_scoped_search.py): each scope narrows correctly, field=all is
  identical to today, episode empties the grid and needs shows in scope.

Frontend:
- PlexSearchBox wraps the shared SearchBar with the scope-pill (reusing
  ActionMenu) and the typeahead; SearchBar gains generic leading/below/focus/
  keydown slots so other modules' search boxes are untouched.
- Episode scope offered only when shows are in scope (movie-only hides it),
  healed at the point of use like the sort state.
- Typeahead reuses /api/plex/library (limit=10), honours scope+field (not the
  sidebar filters), shows poster+title+year, keyboard nav (arrows/Enter/Esc),
  and jumps straight to a movie's info page or a show's drill-down via a new
  provider jumpTo hand-off (mirrors the playlistOpen pattern).
- `field` persisted per-account; en/hu labels added.

frontend/src/i18n/locales/en/plex.json

@@ -3,6 +3,17 @@
   "backToFeed": "YouTube feed",
   "backToLibrary": "Back to library",
   "searchPlaceholder": "Search Plex…",
+  "searchField": {
+    "aria": "Search scope",
+    "all": "Everywhere",
+    "title": "Titles",
+    "people": "Cast & crew",
+    "summary": "Descriptions",
+    "episode": "Episodes"
+  },
+  "suggest": {
+    "aria": "Search suggestions"
+  },
   "sort": {
     "added": "Recently added",
     "title": "Title",

frontend/src/i18n/locales/hu/plex.json

@@ -3,6 +3,17 @@
   "backToFeed": "YouTube feed",
   "backToLibrary": "Vissza a könyvtárhoz",
   "searchPlaceholder": "Keresés a Plexben…",
+  "searchField": {
+    "aria": "Keresési hatókör",
+    "all": "Mindenhol",
+    "title": "Címek",
+    "people": "Szereplők",
+    "summary": "Leírások",
+    "episode": "Epizódok"
+  },
+  "suggest": {
+    "aria": "Keresési javaslatok"
+  },
   "sort": {
     "added": "Nemrég hozzáadott",
     "title": "Cím",