fix(gallery): clear only Prompt Manager cache entries (#222)

The legacy gallery's Clear cache deleted every localStorage key starting
pm_ or promptManager_ (the workspace theme and the dismissed update notice
among them) and every CacheStorage cache whose name held gallery or
thumbnail, which on ComfyUI's shared origin can belong to another tool.

web/js/cache-ownership.js names what a clear may remove from an inventory
of the real writers: every key Prompt Manager writes is a preference and no
page opens CacheStorage or IndexedDB, so nothing is removed and the toast
says so. A test fails when a new storage writer is not classified. The
server thumbnail clear stays its own request; a refused or unreachable
server changes nothing and is reported, and refused browser storage is
reported as such.

tools/vendor/frontend-coverage-baseline.json

@@ -11,6 +11,12 @@
     "functions": 98,
     "statements": 98
   },
+  "cache-ownership.js": {
+    "lines": 80,
+    "branches": 80,
+    "functions": 80,
+    "statements": 80
+  },
   "classic-node.js": {
     "lines": 98,
     "branches": 79,