feat(plex): add Playlist/Collection browse modes to the header pill

The header library-scope pill gains two entry points — Playlist and Collection —
next to All/Movies/Shows, a second way into each area (the rail filter is the
other). Picking Playlist shows a grid of the user's playlists (a tile opens that
playlist's view); Collection shows a searchable grid of collections (a tile
filters the title grid by that collection). Pure front-end — reuses the existing
plexPlaylists / plexCollections(union) list endpoints.

Two new top-level sub-views (`playlists` / `collections`, bookmarkable as
?v=playlists / ?v=collections) rendered by PlexBrowse via the new plexBrowseLists
surfaces (hue-tiled like the Discover genre tiles). The PlexViewBridge now carries
the view `kind`; PlexScopeToggle is a 5-way that derives its active segment from
it (browse-mode when in one, else scope) and drives the two views through the
bridge — picking a scope from a browse mode returns to the title grid. The
Discover/Browse toggle hides in these modes; Backspace + the search→grid flip
cover them.

Tests: PlexScopeToggle (5-way + browse-mode drive/active), plexSubUrl round-trip
for the two new views. Gate: tsc/lint/knip/prettier + 402 vitest.

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

@@ -27,7 +27,9 @@
     "scopeOpt": {
       "both": "All",
       "movie": "Movies",
-      "show": "Shows"
+      "show": "Shows",
+      "playlists": "Playlists",
+      "collections": "Collections"
     },
     "show": "Show",
     "showOpt": {
@@ -284,6 +286,9 @@
     "label": "View",
     "browse": "Browse"
   },
+  "browse": {
+    "items": "{{count}} items"
+  },
   "discover": {
     "tab": "Discover",
     "continueWatching": "Continue watching",

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

@@ -27,7 +27,9 @@
     "scopeOpt": {
       "both": "Mind",
       "movie": "Filmek",
-      "show": "Sorozatok"
+      "show": "Sorozatok",
+      "playlists": "Listák",
+      "collections": "Gyűjtemények"
     },
     "show": "Megjelenítés",
     "showOpt": {
@@ -284,6 +286,9 @@
     "label": "Nézet",
     "browse": "Böngészés"
   },
+  "browse": {
+    "items": "{{count}} elem"
+  },
   "discover": {
     "tab": "Felfedezés",
     "continueWatching": "Folytatás",