refactor(frontend): remove the files-page filter bar; top bar owns filtering

The top bar's unified search box + filter panel (29d0c335) already drives
the same shared filter store, so the files-page SearchFilterBar had become
a second, redundant input. Remove the component and its mount; the shared
store, URL sync and the files page's in-place search mode all stay — the
top bar is now the only filter writer. Drop the orphaned locale keys
(filter.placeholder/keyword/clear_keyword) from all 16 locales and rework
the affected tests to drive the store directly, with a resetFilters() +
URL reset in beforeEach to keep the module-level store from leaking.

Co-Authored-By: Claude Code <noreply@anthropic.com>

frontend/static/locales/it.json

@@ -417,12 +417,9 @@
     "shared_notificationFailed": "Impossibile inviare la notifica"
   },
   "filter": {
-    "placeholder": "Cerca in questa cartella e nelle sottocartelle…",
-    "keyword": "Parola chiave",
     "advanced": "Filtri",
     "recursive": "Includi sottocartelle",
     "results_count": "{{n}} risultati",
-    "clear_keyword": "Cancella ricerca",
     "modified_label": "Modified",
     "created_label": "Created",
     "kind_label": "Items",