fix(plex,channels): address R13 S2 review findings

- Shared usePlexClearFilters hook replaces the duplicated clearAll (was copied in
  PlexBrowse and PlexSidebar) — one "reset every Plex narrowing" both call.
- The Plex duration chip's minutes unit is localized (plex.filter.durationUnit:
  "min"/"perc") instead of a hardcoded "m".
- The Plex collection chip falls back to the "Collection" label, not the raw
  rating_key id, when collectionTitle is missing.
- Dropped the dead rowClassName that dimmed hidden rows in the subscribed table
  (hidden channels are excluded there now — they live in the Hidden tab).
- Renamed the misleading `set` null-check predicate to `has`.

Gate green: tsc/eslint-0/prettier/knip clean, vitest 106.

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

@@ -33,6 +33,7 @@
     "genre": "Genre",
     "year": "Year",
     "duration": "Length",
+    "durationUnit": "min",
     "durationOpt": {
       "short": "Under 90m",
       "mid": "90–120m",

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

@@ -33,6 +33,7 @@
     "genre": "Műfaj",
     "year": "Év",
     "duration": "Hossz",
+    "durationUnit": "perc",
     "durationOpt": {
       "short": "90 perc alatt",
       "mid": "90–120 perc",