feat(plex): P1.5 — Plex as a nav module + integrated search + filter sidebar

Per user feedback: promote Plex from a hidden feed-Source option to a first-class
left-nav module, with its own filter sidebar and the shared top search box.

- Plex is now a nav-rail module (page='plex', gated on me.plex_enabled) — its own
  page → correct browser Back/Forward; removed the Feed Source-dropdown 'plex' hack.
- PlexSidebar.tsx: left filter column (library scope, watch-state for movies:
  all/unwatched/in_progress/watched, sort) — per-account persisted (App state).
- Header search box now also serves the Plex page (integrated search); the live
  YouTube-search escalation stays feed-only.
- PlexBrowse.tsx reworked: infinite scroll (IntersectionObserver, no 'Load more'),
  content-visibility for smoother long-list scroll, show drill-down rides history
  (useHistorySubview) so Back returns to the grid; dropped the stray 'YouTube feed'
  button on the show page.
- backend /browse gains a per-user watch-state filter (show=, movies only).
- plex i18n (navLabel + filter.*) en/hu/de.

Note: episode-title 'Episode N' on some shows (e.g. Westworld) is Plex-side
(unmatched show) — we mirror what Plex has; Match/Refresh in Plex + re-sync fixes it.

backend/app/routes/plex.py

@@ -8,8 +8,8 @@ catalog sync.
 import logging
 
 from fastapi import APIRouter, Depends, HTTPException, Query, Response
-from sqlalchemy import func
-from sqlalchemy.orm import Session
+from sqlalchemy import and_, func, or_
+from sqlalchemy.orm import Session, aliased
 
 from app import sysconfig
 from app.auth import current_user
@@ -130,14 +130,15 @@ def browse(
     library: str,
     q: str | None = None,
     sort: str = "added",
+    show: str = "all",
     offset: int = 0,
     limit: int = Query(default=40, ge=1, le=100),
     user: User = Depends(current_user),
     db: Session = Depends(get_db),
 ) -> dict:
-    """List a library's movies (leaves) or shows, with optional FTS search + sorting.
-    Movie libraries return playable movie cards; show libraries return show cards (drill in via
-    /show/{id})."""
+    """List a library's movies (leaves) or shows, with optional FTS search + sorting + a per-user
+    watch-state filter (`show`: all|unwatched|in_progress|watched — movie libraries only). Movie
+    libraries return playable movie cards; show libraries return show cards (drill in via /show)."""
     lib = db.query(PlexLibrary).filter_by(plex_key=str(library)).first()
     if lib is None:
         raise HTTPException(status_code=404, detail="Unknown Plex library")
@@ -147,6 +148,17 @@ def browse(
     query = db.query(model)
     if lib.kind == "movie":
         query = query.filter(PlexItem.library_id == lib.id, PlexItem.kind == "movie")
+        # Per-user watch-state filter (movies only; a show isn't a single watch unit).
+        st = aliased(PlexState)
+        query = query.outerjoin(st, and_(st.item_id == PlexItem.id, st.user_id == user.id))
+        if show == "watched":
+            query = query.filter(st.status == "watched")
+        elif show == "in_progress":
+            query = query.filter(st.position_seconds > 0, or_(st.status.is_(None), st.status != "watched"))
+        elif show == "unwatched":
+            query = query.filter(or_(st.status.is_(None), st.status.notin_(["watched", "hidden"])))
+        else:  # all — hide only the explicitly hidden
+            query = query.filter(or_(st.status.is_(None), st.status != "hidden"))
     else:
         query = query.filter(PlexShow.library_id == lib.id)